dygraphs JavaScript Library

dygraphs JavaScript Library
code.google.com/p/dygraphs

The dygraphs JavaScript library produces produces interactive, zoomable charts of time series.

Features

Plots time series without using an external server or Flash
Supports multiple data series
Supports error bands around data series
Displays values on mouseover
Interactive zoom
Adjustable averaging period
Customizable click-through actions
Compatible with the Google Visualization API

Caveats

Requires Firefox 1.5+ or Safari/WebKit 1.3+.
Internet Explorer is poorly supported.

Demo

(Mouse over to highlight individual values. Click and drag to zoom. Double-click to zoom out.)

Usage

The dygraphs library depends on two other JS libraries: MochiKit and PlotKit. Rather than tracking down copies of these libraries, I recommend using a packed version of dygraphs that combines all three libraries into a single JS file. Either grab this file from dygraph project's downloads page or create it yourself by checking out a copy of the code and running:

./generate-combined.sh

The combined JS file is now in dygraph-combined.js. Here's a basic example to get things started:

HTML		Output
<html> <head> <script type="text/javascript" src="combined.js"></script> </head> <body> <div id="graphdiv"></div> <script type="text/javascript"> g = new Dygraph( document.getElementById("graphdiv"), // containing div "Date,Temperature\n" + // CSV or path to a CSV file. "20080507,75\n" + "20080508,70\n" + "20080509,80\n", ); </script> </body> </html>

HTML

Output

<html>
<head>
<script type="text/javascript" src="combined.js"></script>
</head>
<body>
<div id="graphdiv"></div>
<script type="text/javascript">
  g = new Dygraph(
        document.getElementById("graphdiv"),  // containing div
        "Date,Temperature\n" +                // CSV or path to a CSV file.
        "20080507,75\n" +
        "20080508,70\n" +
        "20080509,80\n",
      );
</script>
</body>
</html>

In order to keep this example self-contained, the second parameter is a function that returns CSV data. These lines must begin with a date in the form YYYYMMDD. In most applications, it makes more sense to include a CSV file instead. If the second parameter to the constructor is a string, 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="combined.js"></script> </head> <body> <div id="graphdiv" style="width:600px; height:300px;"></div> <script type="text/javascript"> g = new Dygraph( document.getElementById("graphdiv"), "temperatures.csv", // path to CSV file {} // additional options ); </script> </body> </html>

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">
  g = new Dygraph(
        document.getElementById("graphdiv"),
        "temperatures.csv",  // path to CSV file
        {}                   // additional options
      );
</script>
</body>
</html>

Click here to view the temperatures.csv file. 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		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"> g = new Dygraph( document.getElementById("graphdiv"), "temperatures.csv", { rollPeriod: 7, showRoller: true, } ); </script> </body> </html>

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">
  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).

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:800px; height:400px;" ></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>

HTML

Output

<html>
<head>
<script type="text/javascript"
  src="combined.js"></script>
</head>
<body>
<div id="graphdiv" 
 style="width:800px; height:400px;"
 ></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:

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.

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.

Name	Sample Value	Description
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).
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.

Any options you specify also get passed on to PlotKit's Renderer class. dygraphs will override some of these (e.g. strokeColor), but others may be useful. The padding property is an example of this.

Common Gotchas

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

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.

Created May 9, 2008 by Dan Vanderkam