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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.)

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

<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:

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( (σ12 + σ22 + ... + σn2) / 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:480px; height:320px;"></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:

Internet Explorer Compatibility

The dygraphs library relies heavily on the HTML5 <canvas> tag, which Microsoft Internet Explorer did not traditionally support. To use Microsoft's native canvas implementation in IE9, you need to set an HTML5 doctype on your page:

<!DOCTYPE html>

When IE9 is in HTML5 mode, dygraphs works just like in other modern browsers.

If you want to support previous versions of Internet Explorer (IE6–IE8), you'll need to include the excanvas library, which emulates the <canvas> tag using VML. You can add excanvas by including the following snippet:

<!DOCTYPE html> 
<html>
  <head>
    <meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7; IE=EmulateIE9"> 
    <!--[if IE]><script src="path/to/excanvas.js"></script><![endif]-->
  </head>

(This is surprisingly tricky because the HTML5 doctype breaks excanvas in IE8. See this discussion for details.)

While VML emulation sounds like it would be slow, it works well in practice for most charts.

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:

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.

Display:

Other Options

In addition to the options mentioned above (showRoller, rollPeriod, errorBars, valueRange), there are many others.

For a full list, see the Dygraphs Options Reference page.

Common Gotchas

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

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