dygraphs JavaScript Visualization Library
http://github.com/danvk/dygraphs
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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:

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:

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 Output
<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>

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.

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

Click here to view the temperatures.csv file. There are a few things to note here:

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

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_1^2 + σ_2^2 + ... + σ_n^2)/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 here.

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

Things to note here:

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 imlements 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.

For a simple demonstration of how to use dygraphs a GViz visualization, see this page. 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:

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:

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.

Other Options

These are the options that can be passed in through the optional third parameter of the Dygraph constructor. To see demonstrations of many of these options, browse the dygraphs tests directory.

NameSample ValueDescription
includeZero true, 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.
rollPeriod 7 Number of days over which to average data. Discussed extensively above.
showRoller true Should the rolling average period text box be shown? Default is false.
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.
colorSaturation 1.0 If colors is not specified, saturation of the automatically-generated data series colors. (0.0-1.0, default: 1.0)
colorValue 0.5 If colors is not specified, value of the data series colors, as in hue/saturation/value. (0.0-1.0, default 0.5)
clickCallback function(e,date){ alert(date); } 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)
zoomCallback function(minDate,maxDate) {} A function to call when the zoom window is changed (either by zooming in or out). minDate and maxDate are millis since epoch.
strokeWidth 2.0 Width of the data lines. This can be used to increase the contrast or some graphs. (default 1.0)
dateWindow [Date.parse('2006-01-01'),
(new Date()).valueOf()]		 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.
valueRange [10, 110] Explicitly set the vertical range of the graph to [low, high]. By default, some clever heuristics are used (see above).
labelsSeparateLines true Put <br/> between lines in the label string. Often used in conjunction with labelsDiv. (default false)
labelsDiv document.getElementById('foo') Show data labels in an external div, rather than on the graph. (default null)
labelsKMB true Show K/M/B for thousands/millions/billions on y-axis (default false).
labelsDivWidth 250 Width (in pixels) of the div which shows information on the currently-highlighted points.
labelsDivStyles {} Additional styles to apply to the currently-highlighted points div. For example, { 'font-weigth': 'bold' } will make the labels bold.
highlightCircleSize 3 Size (in pixels) of the dot drawn over highlighted points (default 3).
drawPoints 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. Default: false
pointSize 1.0 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.
pixelsPerXLabel, pixelsPerYLabel 50 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).
xAxisLabelWidth, yAxisLabelWidth 50 Width (in pixels) of the x- and y-axis labels.
axisLabelFontSize 14 Size of the font (in pixels) to use in the axis labels, both x- and y-axis.
rightGap 5 Number of pixels to leave blank at the right edge of the Dygraph. This makes it easier to highlight the right-most data point.
errorBars false Does the data contain standard deviations? Setting this to true alters the input format (see above). (default false)
sigma 2 When errorBars is set, shade this many standard deviations above/below each point.
fractions 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).
wilsonInterval 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.
customBars 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.

Common Gotchas

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

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