The dygraphs JavaScript library produces produces interactive, zoomable charts of time series. It is designed to display dense data sets and enable users to explore and interpret them.
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Features
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Plots time series without using an external server or Flash
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Supports error bands around data series
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Displays values on mouseover (this makes it easily discoverable)
There's less seasonal temperature variation in SF than in NY.
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The difference is about 15° F for SF vs. 50° F for NY.
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The daily data (set rolling period to 1) is quite noisy and hides this conclusion.
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Using a 14-day moving average makes it clearer. A 100-day rolling period averages out nearly all the specifics from the data.
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There's a gap in the data for SF, when the weather station was down (zoom into October 2007 to see it).
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The bands around each point indicate average highs and lows.
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There is a lot of data in this chart: low, average and high for each city on each day of a three year period ≈ 6000 data points in all.
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dygraphs allows the user to explore the data and discover these facts.
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For more demos, browse the dygraph tests directory.
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Features
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Some of the features of dygraphs:
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Plots time series without using an external server or Flash
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Works in Internet Explorer (using excanvas)
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Lightweight (45kb) and responsive
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Displays values on mouseover (this makes it easily discoverable)
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Supports error bands around data series
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Interactive zoom
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Adjustable averaging period
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Can intelligently chart fractions
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Customizable click-through actions
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Compatible with the Google Visualization API
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Intelligent defaults make it easy to use
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Usage
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To use dygraphs, include the dygraph-combined.js JavaScript file and instantiate a Dygraph object.
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Here's a basic example to get things started:
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HTML
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<html>
<head>
-<script type="text/javascript"
- src="dygraph-combined.js"></script>
+<script type="text/javascript"
+ src="dygraph-combined.js"></script>
</head>
<body>
-<div id="graphdiv"></div>
-<script type="text/javascript">
+<div id="graphdiv"></div>
+<script type="text/javascript">
g = new Dygraph(
- // containing div
- document.getElementById("graphdiv"),
- // CSV or path to a CSV file.
- "Date,Temperature\n" +
- "2008-05-07,75\n" +
- "2008-05-08,70\n" +
- "2008-05-09,80\n"
- );
+
+ // containing div
+ document.getElementById("graphdiv"),
+
+ // CSV or path to a CSV file.
+ "Date,Temperature\n" +
+ "2008-05-07,75\n" +
+ "2008-05-08,70\n" +
+ "2008-05-09,80\n"
+
+ );
</script>
</body>
</html>
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In order to keep this example self-contained, the second parameter is raw CSV data. The dygraphs library parses this data (including column headers), resizes the its container to a reasonable default, calculates appropriate axis ranges and tick marks and draws the graph.
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In most applications, it makes more sense to include a CSV file instead. If the second parameter to the constructor doesn't contain a newline, it will be interpreted as the path to a CSV file. The Dygraph will perform an XMLHttpRequest to retrieve this file and display the data when it becomes available. Make sure your CSV file is readable and serving from a place that understands XMLHttpRequest's! In particular, you cannot specify a CSV file using "file:///". Here's an example: (data from Weather Underground)
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HTML
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Output
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OUTPUT
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In order to keep this example self-contained, the second parameter is raw CSV data. The dygraphs library parses this data (including column headers), resizes its container to a reasonable default, calculates appropriate axis ranges and tick marks and draws the graph.
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In most applications, it makes more sense to include a CSV file instead. If the second parameter to the constructor doesn't contain a newline, it will be interpreted as the path to a CSV file. The Dygraph will perform an XMLHttpRequest to retrieve this file and display the data when it becomes available. Make sure your CSV file is readable and serving from a place that understands XMLHttpRequest's! In particular, you cannot specify a CSV file using "file:///". Here's an example: (data from Weather Underground)
Click here to view the temperatures.csv file. There are a few things to note here:
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The Dygraph sent off an XHR to get the temperatures.csv file.
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The labels were taken from the first line of temperatures.csv, which is Date,High,Low.
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The Dygraph automatically chose two different, easily-distinguishable colors for the two data series.
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The labels on the x-axis have switched from days to months. If you zoom in, they'll switch to weeks and then days.
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Some heuristics are used to determine a good vertical range for the data. The idea is to make all the data visible and have human-friendly values on the axis (i.e. 200 instead of 193.4). Generally this works well.
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The data is very spiky. A moving average would be easier to interpret.
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This problem can be fixed by specifying the appropriate options in the "additional options" parameter to the Dygraph constructor. To set the number of days for a moving average, use the rollPeriod option. Here's how it's done:
The Dygraph sent off an XHR to get the temperatures.csv file.
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The labels were taken from the first line of temperatures.csv, which is Date,High,Low.
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The Dygraph automatically chose two different, easily-distinguishable colors for the two data series.
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The labels on the x-axis have switched from days to months. If you zoom in, they'll switch to weeks and then days.
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Some heuristics are used to determine a good vertical range for the data. The idea is to make all the data visible and have human-friendly values on the axis (i.e. 200 instead of 193.4). Generally this works well.
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The data is very spiky. A moving average would be easier to interpret.
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This problem can be fixed by specifying the appropriate options in the "additional options" parameter to the Dygraph constructor. To set the number of days for a moving average, use the rollPeriod option. Here's how it's done:
A rolling average can be set using the text box in the lower left-hand corner of the graph (the showRoller attribute is what makes this appear). Also note that we've explicitly set the size of the chart div.
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Error Bars
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Another significant feature of the dygraphs library is the ability to display error bars around data series. One standard deviation must be specified for each data point. A +/-n sigma band will be drawn around the data series at that point. If a moving average is being displayed, dygraphs will compute the standard deviation of the average at each point. (i.e. σ = sqrt((σ_1^2 + σ_2^2 + ... + σ_n^2)/n))
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Here's a demonstration. There are two data series. One is N(100,10) with a standard deviation of 10 specified at each point. The other is N(80,20) with a standard deviation of 20 specified at each point. The CSV file was generated using Octave and can be viewed here.
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HTML
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Output
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OUTPUT
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A rolling average can be set using the text box in the lower left-hand corner of the graph (the showRoller attribute is what makes this appear). Also note that we've explicitly set the size of the chart div.
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Error Bars
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Another significant feature of the dygraphs library is the ability to display error bars around data series. One standard deviation must be specified for each data point. A ±n sigma band will be drawn around the data series at that point. If a moving average is being displayed, dygraphs will compute the standard deviation of the average at each point. I.E. σ = sqrt( (σ12 + σ22 + ... + σn2) / n )
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Here's a demonstration. There are two data series. One is N(100,10) with a standard deviation of 10 specified at each point. The other is N(80,20) with a standard deviation of 20 specified at each point. The CSV file was generated using Octave and can be viewed at twonormals.csv.
The errorBars option affects both the interpretation of the CSV file and the display of the graph. When errorBars is set to true, each line is interpreted as YYYYMMDD,A,sigma_A,B,sigma_B,...
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The first line of the CSV file doesn't mention the error columns. In this case, it's just "Date,Series1,Series2".
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The averaging visibly affects the error bars. This is most clear if you crank up the rolling period to something like 100 days. For the earliest dates, there won't be 100 data points to average so the signal will be noisier. The error bars get smaller like sqrt(N) going forward in time until there's a full 100 points to average.
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The error bars are partially transparent. This can be seen when they overlap one another.
The errorBars option affects both the interpretation of the CSV file and the display of the graph. When errorBars is set to true, each line is interpreted as YYYYMMDD, A, sigma_A, B, sigma_B, …
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The first line of the CSV file doesn't mention the error columns. In this case, it's just "Date,Series1,Series2".
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The averaging visibly affects the error bars. This is most clear if you crank up the rolling period to something like 100 days. For the earliest dates, there won't be 100 data points to average so the signal will be noisier. The error bars get smaller like sqrt(N) going forward in time until there's a full 100 points to average.
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The error bars are partially transparent. This can be seen when they overlap one another.
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Internet Explorer Compatibility
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The dygraphs library relies heavily on HTML's <canvas> tag, which Microsoft Internet Explorer does not support. Fortunately, some clever engineers created the excanvas library, which implements the <canvas> tag in IE using VML.
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You can add IE support to any page using dygraphs by including the following in your page:
Most browsers will ignore the trailing comma, but it will break under IE.
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GViz Data
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The Google Visualization API provides a standard interface for describing data. Once you've specified your data using this API, you can plug in any GViz-compatible visualization. dygraphs is such a visualization.
Here's an example of a published gviz gadget using dygraphs:
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Charting Fractions
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Situations often arise where you want to plot fractions, e.g. the fraction of respondents in a poll who said they'd vote for candidate X or the number of hits divided by at bats (baseball's batting average). Fractions require special treatment for two main reasons:
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The average of a1/b1 and a2/b2 is (a1+a2)/(b1+b2), not (a1/b1 + a2/b2)/2.
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The normal approximation is not always applicable and more sophisticated confidence intervals (e.g. the Wilson confidence interval) must be employed to avoid ratios that exceed 100% or go below 0%.
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Fortunately, dygraphs handles both of these for you! Here's a chart and the command that generated it:
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Batting Average for Ichiro Suzuki vs. Mariners (2004)
The fractions option indicates that the values in each column should be parsed as fractions (e.g. "1/2" instead of "0.5"). The errorBars option indicates that we'd like to see a confidence interval around each data point. By default, when fractions is set, you get a Wilson confidence interval. If you look carefully at the chart, you can see that the error bars are asymmetric.
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A couple things to notice about this chart:
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The error bars for Ichiro's batting average are larger than for the Mariners', since he has far fewer at bats than his team.
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dygraphs makes it easy to see "batting average over the last 30 games". This is ordinarily quite difficult to compute. It makes it clear where the "hot" and "cold" part of Suzuki's season were.
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If you set the averaging period to something large, like 200, you'll see the team's and player's batting average through that game. The final number is the overall batting average for the season.
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Where the error bars do not overlap, we can say with 95% confidence that the series differ. There is a better than 95% chance that Ichiro was a better hitter than his team as a whole in 2004, the year he won the batting title.
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One last demo
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This chart shows monthly closes of the Dow Jones Industrial Average, both in nominal and real (i.e. adjusted for inflation) dollars. The shaded areas show its monthly high and low. CPI values with a base from 1982-84 are used to adjust for inflation.
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Display:
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Other Options
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These are the options that can be passed in through the optional third parameter of the Dygraph constructor. Under each option is a set of links to tests which demonstrate its use. While we attempt to keep this list up-to-date, a more complete list can be found in the Dygraph.DEFAULT_ATTRS map in dygraph.js and the this.options map in dygraph-canvas.js.
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Name
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Values
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Default
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Description
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includeZero
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boolean
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false
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Usually, dygraphs will use the range of the data plus some padding to set the range of the y-axis. If this option is set, the y-axis will always include zero, typically as the lowest value. This can be used to avoid exaggerating the variance in the data.
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List of colors for the data series. These can be of the form "#AABBCC" or "rgb(255,100,200)" or "yellow", etc. If not specified, equally-spaced points around a color wheel are used.
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Which series should initially be visible? Once the Dygraph has
+ been constructed, you can access and modify the visibility of each
+ series using the visibility and
+ setVisibility methods.
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If colors is not specified, saturation of the automatically-generated data series colors.
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Tests: none
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colorValue
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float (0.0 — 1.0)
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1.0
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If colors is not specified, value of the data series colors, as in hue/saturation/value. (0.0-1.0, default 0.5)
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Tests: none
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clickCallback
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function(e, date){ alert(date); }
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null
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A function to call when a data point is clicked. The function should take two arguments, the event object for the click and the date that was clicked. (default null)
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Draw a small dot at each point, in addition to a line going through the point. This makes the individual data points easier to see, but can increase visual clutter in the chart.
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The size of the dot to draw on each point in pixels (see drawPoints). A dot is always drawn when a point is "isolated", i.e. there is a missing point on either side of it. This also controls the size of those dots.
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Tests: none
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pixelsPerXLabel
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integer
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60
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Number of pixels to require between each x- and y-label. Larger values will yield a sparser axis with fewer ticks.
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When errorBars is set, shade this many standard deviations above/below each point.
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Tests: none
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fractions
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boolean
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false
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When set, attempt to parse each cell in the CSV file as "a/b", where a and b are integers. The ratio will be plotted. This allows computation of Wilson confidence intervals (see below).
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Use in conjunction with the "fractions" option. Instead of plotting +/- N standard deviations, dygraphs will compute a Wilson confidence interval and plot that. This has more reasonable behavior for ratios close to 0 or 1.
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Tests: none
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customBars
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boolean
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false
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When set, parse each CSV cell as "low;middle;high". Error bars will be drawn for each point between low and high, with the series itself going through middle.
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When set, this callback gets called every time the dygraph is drawn. This includes the initial draw, after zooming and repeatedly while panning. The first parameter is the dygraph being drawn. The second is a boolean value indicating whether this is the initial draw.
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When set, this callback gets called every time a new point is highlighted. The parameters are the JavaScript mousemove event, the x-coordinate of the highlighted points and an array of highlighted points: [ {name: 'series', yval: y-value}, … ]
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When set, this callback gets called every time the user stops highlighting any point by mousing out of the graph. The parameter is the mouseout event.
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Here are a few problems that I've frequently run into while using the dygraphs library.
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If your chart doesn't display, be sure to check your browser's JavaScript error console. dygraphs makes every attempt to log errors and warnings, and these can often guide you in the right direction.
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Make sure your CSV files are readable! If your graph isn't showing up, the XMLHttpRequest for the CSV file may be failing. You can determine whether this is the case using tools like Firebug.
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Make sure your CSV files are in the correct format. They must be of the form YYYYMMDD, series1, series2, … . And if you set the errorBars property, make sure you alternate data series and standard deviations.
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dygraphs are not happy when placed inside a <center> tag. This applies to the CSS text-align property as well. If you want to center a Dygraph, put it inside a table with align = center set.
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Don't set the dateWindow property to a date. It expects milliseconds since epoch, which can be obtained from a JavaScript Date object's valueOf method.
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Make sure you don't have any trailing commas in your call to the Dygraph constructor or in the options parameter. Firefox, Chrome and Safari ignore these but they can cause a graph to not display in Internet Explorer.
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GWT Compatibility
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There is currently no GWT wrapper around Dygraphs, however there is a class that can be used to easily load Dygraphs into the browser. To use it, include the generated dygraph-gwt.jar file in your classpath and add the following line to your GWT module:
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+<inherits name="org.danvk.dygraphs"/>
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Call org.danvk.Dygraphs.install() when your application starts to install the JavaScript code into the browser. You can use JSNI to call Dygraphs from your GWT code, as in the example below. The example uses the Visualization API for GWT and the Dygraphs GViz API.
Usually, dygraphs will use the range of the data plus some padding to
- set the range of the y-axis. If this option is set, the y-axis will always
- include zero, typically as the lowest value. This can be used to avoid
- exaggerating the variance in the data.
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rollPeriod
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7
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Number of days over which to average data. Discussed extensively above.
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showRoller
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true
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Should the rolling average period text box be shown? Default is false.
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colors
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['red', '#00FF00']
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List of colors for the data series. These can be of the form "#AABBCC"
- or "rgb(255,100,200)" or "yellow", etc. If not specified, equally-spaced
- points around a color wheel are used.
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colorSaturation
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1.0
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If colors is not specified, saturation of the
- automatically-generated data series colors. (0.0-1.0, default:
- 1.0)
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colorValue
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0.5
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If colors is not specified, value of the data series colors, as in
- hue/saturation/value. (0.0-1.0, default 0.5)
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clickCallback
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function(e,date){ alert(date); }
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A function to call when a data point is clicked. The function should take
- two arguments, the event object for the click and the date that was
- clicked. (default null)
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zoomCallback
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function(minDate,maxDate) {}
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A function to call when the zoom window is changed (either by zooming
- in or out). minDate and maxDate are millis since epoch.
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strokeWidth
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2.0
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Width of the data lines. This can be used to increase the contrast or
- some graphs. (default 1.0)
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dateWindow
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[Date.parse('2006-01-01'),
- (new Date()).valueOf()]
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Initially zoom in on a section of the graph. Is of the form [earliest,
- latest], where earliest/latest are millis since epoch. By default, the
- full range of the input is shown.
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valueRange
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[10, 110]
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Explicitly set the vertical range of the graph to [low, high]. By
- default, some clever heuristics are used (see above).
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labelsSeparateLines
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true
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Put <br/> between lines in the label string. Often used in
- conjunction with labelsDiv. (default false)
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labelsDiv
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document.getElementById('foo')
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Show data labels in an external div, rather than on the graph. (default
- null)
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labelsKMB
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true
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Show K/M/B for thousands/millions/billions on y-axis (default
- false).
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labelsDivWidth
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250
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Width (in pixels) of the div which shows information on the
- currently-highlighted points.
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labelsDivStyles
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{}
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Additional styles to apply to the currently-highlighted points div. For
- example, { 'font-weigth': 'bold' } will make the labels bold.
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highlightCircleSize
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3
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Size (in pixels) of the dot drawn over highlighted points (default 3).
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drawPoints
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false
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Draw a small dot at each point, in addition to a line going through
- the point. This makes the individual data points easier to see, but can
- increase visual clutter in the chart. Default: false
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pointSize
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1.0
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The size of the dot to draw on each point in pixels (see
- drawPoints). A dot is always drawn when a point is "isolated", i.e.
- there is a missing point on either side of it. This also controls the
- size of those dots.
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pixelsPerXLabel, pixelsPerYLabel
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50
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Number of pixels to require between each x- and y-label. Larger values
- will yield a sparser axis with fewer ticks. Defaults: 60 (x-axis), 30
- (y-axis).
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xAxisLabelWidth, yAxisLabelWidth
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50
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Width (in pixels) of the x- and y-axis labels.
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axisLabelFontSize
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14
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Size of the font (in pixels) to use in the axis labels, both x- and
- y-axis.
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rightGap
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5
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Number of pixels to leave blank at the right edge of the Dygraph. This
- makes it easier to highlight the right-most data point.
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errorBars
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false
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Does the data contain standard deviations? Setting this to true alters
- the input format (see above). (default false)
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sigma
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2
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When errorBars is set, shade this many standard deviations above/below
- each point.
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fractions
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false
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When set, attempt to parse each cell in the CSV file as "a/b", where a
- and b are integers. The ratio will be plotted. This allows computation of
- Wilson confidence intervals (see below).
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-
-
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wilsonInterval
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true
-
Use in conjunction with the "fractions" option. Instead of plotting +/-
- N standard deviations, dygraphs will compute a Wilson confidence interval
- and plot that. This has more reasonable behavior for ratios close to 0 or
- 1.
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customBars
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false
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When set, parse each CSV cell as "low;middle;high". Error bars will be
- drawn for each point between low and high, with the series itself going
- through middle.
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Common Gotchas
-
Here are a few problems that I've frequently run into while using the
-dygraphs library.
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-
-
Make sure your CSV files are readable! If your graph isn't showing up,
- the XMLHttpRequest for the CSV file may be failing. You can determine whether
- this is the case using tools like Firebug.
-
-
Make sure your CSV files are in the correct format. They must be of the
- form YYYYMMDD,series1,series2,.... And if you set the
- errorBars property, make sure you alternate data series and
- standard deviations.
-
-
dygraphs are not happy when placed inside a <center>
- tag. This applies to the CSS text-align property as well. If you
- want to center a Dygraph, put it inside a table with "align=center"
- set.
-
-
Don't set the dateWindow property to a date. It expects
- milliseconds since epoch, which can be obtained from a JavaScript Date
- object's valueOf method.
-
-
Make sure you don't have any trailing commas in your call to the Dygraph
- constructor or in the options parameter. Firefox, Chrome and Safari ignore
- these but they can cause a graph to not display in Internet Explorer.
