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

<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 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

<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: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:

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.

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.

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.

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.
rollPeriod integer >= 1 1 Number of days over which to average data. Discussed extensively above.
showRoller boolean false If the rolling average period text box should be shown.
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 0.0 - 1.0 1.0 If colors is not specified, saturation of the automatically-generated data series colors.
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)
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)
zoomCallback function(minDate, maxDate){} null A function to call when the zoom window is changed (either by zooming in or out). minDate and maxDate are milliseconds since epoch.
strokeWidth integer 1 Width of the data lines. This can be used to increase the contrast or some graphs.
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.
valueRange [10, 110] Explicitly set the vertical range of the graph to [low, high].
labelsSeparateLines boolean false Put <br/> between lines in the label string. Often used in conjunction with labelsDiv.
labelsDiv document.getElementById('foo') null Show data labels in an external div, rather than on the graph.
labelsKMB true false Show K/M/B for thousands/millions/billions on y-axis.
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.
labelsDivWidth 250 Width (in pixels) of the div which shows information on the currently-highlighted points.
labelsDivStyles {} null Additional styles to apply to the currently-highlighted points div. For example, { 'font-weight': 'bold' } will make the labels bold.
highlightCircleSize integer 3 The size in pixels of the dot drawn over highlighted points.
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.
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.
pixelsPerXLabel integer 60 Number of pixels to require between each x- and y-label. Larger values will yield a sparser axis with fewer ticks.
pixelsPerYLabel 30
xAxisLabelWidth integer Width (in pixels) of the x- and y-axis labels.
yAxisLabelWidth
axisLabelFontSize integer 14 Size of the font (in pixels) to use in the axis labels, both x- and y-axis.
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.
errorBars boolean false Does the data contain standard deviations? Setting this to true alters the input format (see above).
sigma integer When errorBars is set, shade this many standard deviations above/below each point.
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).
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.
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.

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