dygraphs JavaScript Visualization Library

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dygraphs is an open source JavaScript library that produces produces interactive, zoomable charts of time series. It is designed to display dense data sets and enable users to explore and interpret them.

A demo is worth a thousand words:

(Mouse over to highlight individual values. Click and drag to zoom. Double-click to zoom back out. Change the number and hit enter to adjust the averaging period.)

Temperatures in New York vs. San Francisco

Some things to notice:

There's less seasonal temperature variation in SF than in NY.
The difference is about 15° F for SF vs. 50° F for NY.
The daily data (set rolling period to 1) is quite noisy and hides this conclusion.
Using a 14-day moving average makes it clearer. A 100-day rolling period averages out nearly all the specifics from the data.
There's a gap in the data for SF, when the weather station was down (zoom into October 2007 to see it).
The bands around each point indicate average highs and lows.
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.

dygraphs allows the user to explore the data and discover these facts.

For more demos, browse the dygraph tests directory.

Features

Some of the features of dygraphs:

Plots time series without using an external server or Flash
Works in Internet Explorer (using excanvas)
Lightweight (57kb) and responsive
Displays values on mouseover (this makes it easily discoverable)
Supports error bands around data series
Interactive zoom
Displays Annotations on the chart
Adjustable averaging period
Can intelligently chart fractions
Customizable click-through actions
Compatible with the Google Visualization API
Intelligent defaults make it easy to use

Usage

To use dygraphs, include the dygraph-combined.js JavaScript file and instantiate a Dygraph object.

Here's a basic example to get things started:

          HTML
          
  <html>
<head>
<script type="text/javascript"
  src="dygraph-combined.js"></script>
</head>
<body>
<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"

  );
</script>
</body>
</html>

          
        

OUTPUT

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.

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)

          HTML
          
<html>
<head>
<script type="text/javascript"
  src="dygraph-combined.js"></script>
</head>
<body>
<div id="graphdiv2"
  style="width:500px; height:300px;"></div>
<script type="text/javascript">
  g2 = new Dygraph(
    document.getElementById("graphdiv2"),
    "temperatures.csv", // path to CSV file
    {}          // options
  );
</script>
</body>
</html>

          
        

OUTPUT

The file used is temperatures.csv.

There are a few things to note here:

The Dygraph sent off an XHR to get the temperatures.csv file.
The labels were taken from the first line of temperatures.csv, which is Date,High,Low.
The Dygraph automatically chose two different, easily-distinguishable colors for the two data series.
The labels on the x-axis have switched from days to months. If you zoom in, they'll switch to weeks and then days.
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.
The data is very spiky. A moving average would be easier to interpret.

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:

          HTML
          
<html>
<head>
<script type="text/javascript"
  src="dygraph-combined.js"></script>
</head>
<body>
<div id="graphdiv3"
  style="width:500px; height:300px;"></div>
<script type="text/javascript">
  g3 = new Dygraph(
    document.getElementById("graphdiv3"),
    "temperatures.csv",
    {
      rollPeriod: 7,
      showRoller: true
    }
  );
</script>
</body>
</html>

          
        

OUTPUT

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.

Error Bars

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_1² + σ_2² + ... + σ_n²) / n )

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.

          HTML
          
<html>
<head>
<script type="text/javascript"
  src="combined.js"></script>
</head>
<body>
<div id="graphdiv4"
  style="width:600px; height:300px;"></div>
<script type="text/javascript">
  g4 = new Dygraph(
    document.getElementById("graphdiv4"),
    "twonormals.csv",
    {
      rollPeriod: 7,
      showRoller: true,
      errorBars: true,
      valueRange: [50,125]
    }
  );
</script>
</body>
</html>

          
        

OUTPUT

Things to note here:

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, …
The first line of the CSV file doesn't mention the error columns. In this case, it's just "Date,Series1,Series2".
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.
The error bars are partially transparent. This can be seen when they overlap one another.

Internet Explorer Compatibility

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.

You can add IE support to any page using dygraphs by including the following in your page:

<head>
  <!--[if IE]><script src="excanvas.js"></script><![endif]-->
</head>

This works quite well in practice. Charts are responsive, even under VML emulation.

One common gotcha to look out for: make sure you don't have any trailing commas in parameter lists, e.g.

new Dygraph(el, data, {
  showRoller: true,  // note trailing comma
})

Most browsers will ignore the trailing comma, but it will break under IE.

GViz Data

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. In particular, it can be used as a drop-in replacement for the AnnotatedTimeline visualization used on Google Finance and other sites. To see how this works, check out the gviz annotation demo.

For a simple demonstration of how to use dygraphs a GViz visualization, see http://danvk.org/dygraphs/tests/gviz.html. dygraphs can also be used as a GViz gadget. This allows it to be embedded inside of a Google Spreadsheet. For a demonstration of this, see this spreadsheet. The URL for the gadget is http://danvk.org/dygraphs/gadget.xml.

Here's an example of a published gviz gadget using dygraphs:

Charting Fractions

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:

The average of a1/b1 and a2/b2 is (a1+a2)/(b1+b2), not (a1/b1 + a2/b2)/2.
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%.

Fortunately, dygraphs handles both of these for you! Here's a chart and the command that generated it:

Batting Average for Ichiro Suzuki vs. Mariners (2004)

Command:

  new Dygraph(
    document.getElementById("baseballdiv"),
    "suzuki-mariners.txt",
    {
      fractions: true,
      errorBars: true,
      showRoller: true,
      rollPeriod: 15
    }
  );

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.

A couple things to notice about this chart:

The error bars for Ichiro's batting average are larger than for the Mariners', since he has far fewer at bats than his team.
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.
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.
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.

One last demo

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.

Display: Nominal Real Annotations

Other Options

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.

Name	Values	Default	Description
includeZero	`boolean`	`false`	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. Tests: no-range numeric-gviz small-range-zero
rollPeriod	`integer >= 1`	`1`	Number of days over which to average data. Discussed extensively above. Tests: demo two-series noise
showRoller	`boolean`	`false`	If the rolling average period text box should be shown. Tests: callback crosshair fractions isolated-points numeric-gviz underlay-callback highlighted-region
colors	`['red', '#00FF00']`		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. Tests: color-visibility demo
fillGraph	`boolean`	`false`	Should the area underneath the graph be filled? This option is not compatible with error bars. Tests: fillGraph
visibility	`Array of booleans`	`[true, true, ...]`	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. Tests: visibility color-visibility
colorSaturation	`0.0 - 1.0`	`1.0`	If colors is not specified, saturation of the automatically-generated data series colors. Tests: none
colorValue	`float (0.0 — 1.0)`	`1.0`	If colors is not specified, value of the data series colors, as in hue/saturation/value. (0.0-1.0, default 0.5) Tests: none
clickCallback	`function(e, date){ alert(date); }`	`null`	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) Tests: callback
zoomCallback	`function(minDate, maxDate, minValue, maxValue){}`	`null`	A function to call when the zoom window is changed (either by zooming in or out). minDate and maxDate are milliseconds since epoch. minValue and maxValue are y-axis range values. Tests: callback zoom
strokeWidth	`integer`	`1`	Width of the data lines. This can be used to increase the contrast or some graphs. Tests: none
dateWindow	`[ Date.parse('2006-01-01'), (new Date()).valueOf() ]`	`Full range of the input is shown`	Initially zoom in on a section of the graph. Is of the form [earliest, latest], where earliest/latest are milliseconds since epoch. Tests: dateWindow link-interaction
valueRange	`[10, 110]`	`Full range of the input is shown`	Explicitly set the vertical range of the graph to [low, high]. Tests: none
labelsSeparateLines	`boolean`	`false`	Put `<br/>` between lines in the label string. Often used in conjunction with labelsDiv. Tests: customLabel demo
labelsDiv	`document.getElementById('foo')` or `'foo'`	`null`	Show data labels in an external div, rather than on the graph. This value can either be a div element or a div id. Tests: label-div
labelsShowZeroValues	`boolean`	`true`	Show zero value labels in the labelsDiv. Tests: label-div
labelsKMB	`true`	`false`	Show K/M/B for thousands/millions/billions on y-axis. Tests: demo labelsKMB no-range
labelsKMG2	`true`	`false`	Show k/M/G for kilo/Mega/Giga on y-axis. This is different than `labelsKMB` in that it uses base 2, not 10. Tests: labelsKMB
labelsDivWidth	`250`		Width (in pixels) of the div which shows information on the currently-highlighted points. Tests: customLabel
labelsDivStyles	`{}`	`null`	Additional styles to apply to the currently-highlighted points div. For example, { 'font-weight': 'bold' } will make the labels bold. Tests: border customLabel
highlightCircleSize	`integer`	`3`	The size in pixels of the dot drawn over highlighted points. Tests: grid_dot
drawPoints	`boolean`	`false`	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. Tests: draw-points
pointSize	`interger`	`1`	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. Tests: none
pixelsPerXLabel	`integer`	`60`	Number of pixels to require between each x- and y-label. Larger values will yield a sparser axis with fewer ticks. Tests: spacing
pixelsPerYLabel	`integer`	`30`
xAxisLabelWidth	`integer`		Width (in pixels) of the x- and y-axis labels. Tests: customLabel
yAxisLabelWidth	`integer`
axisLabelFontSize	`integer`	`14`	Size of the font (in pixels) to use in the axis labels, both x- and y-axis. Tests: none
xAxisLabelFormatter	`function(date, granularity)`	`Dygraph.dateAxisFormatter`	Function to call to format values along the x axis. Tests: xAxisLabelFormatter
yAxisLabelFormatter	`function(x)`	`yValueFormatter`	Function used to format values along the Y axis. By default it uses the same as the `yValueFormatter` unless specified. Tests: yAxisLabelFormatter
rightGap	`integer`		Number of pixels to leave blank at the right edge of the Dygraph. This makes it easier to highlight the right-most data point. Tests: none
errorBars	`boolean`	`false`	Does the data contain standard deviations? Setting this to true alters the input format (see above). Tests: noise customLabel draw-points fillGraph fractions
sigma	`integer`		When errorBars is set, shade this many standard deviations above/below each point. Tests: none
fractions	`boolean`	`false`	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). Tests: fractions
wilsonInterval	`boolean`	`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. Tests: none
customBars	`boolean`	`false`	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. Tests: custom-bars zero-series
drawCallback	`function(dygraph, is_initial)`	`null`	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. Tests: callback synchronize
gridLineColor	`red, blue`	`rgb(128,128,128)`	The color of the gridlines. Tests: grid_dot
highlightCallback	`function(event, x, points,row)`	`null`	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}, … ]` Tests: callback crosshair
unhighlightCallback	`function(event)`	`null`	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. Tests: callback crosshair
underlayCallback	`function(canvas, area, dygraph)`	`null`	When set, this callback gets called before the chart is drawn. It allows you to draw underneath the chart. See the tests for more details on how to use this. Tests: underlay-callback highlighted-region
strokeWidth	`0.5, 2.0`	`1.0`	The width of the lines connecting data points. Tests: grid_dot
width	`integer`	`480`	Width/Height (in pixels) of the chart. If the container div has been explicitly sized, these attributes will be ignored. Tests: demo link-interaction no-range
height	`integer`	`320`
stepPlot	`boolean`	`false`	When set, display the graph as a step plot instead of a line plot. Tests: steps
xValueFormatter	`function(x)`		Function to provide a custom display format the X value for mouseover.
yValueFormatter	`function(x)`	(Round to 2 decimal places)	Function to provide a custom display format for the Y value for mouseover. Tests: yAxisLabelFormatter
avoidMinZero	`boolean`	`false`	When set, the heuristic that fixes the Y axis at zero for a data set with the minimum Y value of zero is disabled. This is particularly useful for data sets that contain many zero values, especially for step plots which may otherwise have lines not visible running along the bottom axis. Tests: avoidMinZero

Common Gotchas

Here are a few problems that I've frequently run into while using the dygraphs library.

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.
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.

GWT Compatibility

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:

<inherits name="org.danvk.dygraphs"/>

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.

  public static native JavaScriptObject drawDygraph(Element element, DataTable dataTable, double minY, double maxY) /*-{
    var chart = new $wnd.Dygraph.GVizChart(element);
    chart.draw(dataTable,
      {
        valueRange: [minY, maxY]
      });
    return chart;
  }-*/;

Data Policy

dygraphs is purely client-side JavaScript. It does not send your data to any servers – the data is processed entirely in the client's browser.

Created May 9, 2008 by Dan Vanderkam

Documentation

Features

Project

Gallery

dygraphs JavaScript Visualization Library

A demo is worth a thousand words:

Some things to notice:

Features

Usage

HTML

OUTPUT

HTML

OUTPUT

HTML

OUTPUT

Error Bars

HTML

OUTPUT

Internet Explorer Compatibility

GViz Data

Charting Fractions

One last demo

Other Options

Common Gotchas

GWT Compatibility

Data Policy