<script type="text/javascript" src="/mochikit/MochiKit.js"></script>
<script type="text/javascript" src="/plotkit/Base.js"></script>
<script type="text/javascript" src="/plotkit/Layout.js"></script>
<script type="text/javascript" src="/plotkit/Canvas.js"></script>
<script type="text/javascript" src="/plotkit/SweetCanvas.js"></script>

tags for labels to work.

 <div><canvas id="graph" height="300" width="300"></canvas></div>

This will create a blank canvas of 300 by 300 pixels.

Javascript

There are only two simple steps to draw a chart, first is the create a layout with our data and second is to create the renderer. So lets start off with creating the layout.

Layout and Data

var layout = new PlotKit.Layout("bar", {});
layout.addDataset("sqrt", [[0, 0], [1, 1], [2, 1.414], [3, 1.73], [4, 2]]);
layout.evaluate();

There, it is that simple. Lets explain what each line is doing:

1. var layout = new PlotKit.Layout("bar", {});

   We create a new layout object, and tell it that we want a bar chart in the first parameter. The second parameter allows us to pass additional options, which we will go on to later. It can be left null, or in this case just an empty array.

2. layout.addDataset("sqrt", [[0, 0], [1, 1]...)

   This will add a new dataset to the layout. You can add multiple datasets by specifying a different name in the first parameter for each dataset. The dataset consists of an array of (x, y) values. These must be numeric, either floating point or integers.

   Note that PlotKit does not deal with negative numbers at the moment.

3. layout.evaluate();

   This will be the last command you make on the layout before passing it to the renderer. This will tell the layout to calculate the layout of the chart so the renderer can draw it. It is an expensive operation, so do not call it frequently, only unless the data or options have been changed.

Renderer

 var canvas = MochiKit.DOM.getElement("graph");
 var plotter = new PlotKit.SweetCanvasRenderer(canvas, layout, {});
 plotter.render();

1. var canvas = MochiKit.DOM.getElement("graph");

   This line will get the HTML element we defined earlier.

2. var plotter = new PlotKit.SweetCanvasRenderer(canvas, layout, {});

   This will create the renderer to work on the object passed, and also with the data in the layout we created earlier. Again, the third parameter here is for options to relates to the look of the graph. We will show you some things you can do with this in the following section.

3. plotter.render()

   This line will render the graph.

Putting it altogether

function drawGraph() {
    var layout = new PlotKit.Layout("bar", {});
    layout.addDataset("sqrt", [[0, 0], [1, 1], [2, 1.414], [3, 1.73], [4, 2]]);
    layout.evaluate();
    var canvas = MochiKit.DOM.getElement("graph");
    var plotter = new PlotKit.SweetCanvasRenderer(canvas, layout, {});
    plotter.render();
}
MochiKit.DOM.addLoadEvent(drawGraph);

This is a sample of what you would use to plot the graph of sqare roots for 0 to 4. Make sure you plot the graph on the load event because the DOM will not be ready if when the Javascript is first loaded.

See this in an [HTML example here][example1].

Additional Options

We mentioned that both the layout and renderer may take some additional options. In order to take advantage of that, we can use a simple options dictionary to store options for both layout and the renderer, in this way:

var options = {
   "IECanvasHTC": "/plotkit/iecanvas.htc",
   "colorScheme": PlotKit.Base.palette(PlotKit.Base.baseColors()[0]),
   "padding": {left: 0, right: 0, top: 10, bottom: 30},
   "xTicks": [{v:0, label:"zero"}, 
          {v:1, label:"one"}, 
          {v:2, label:"two"},
          {v:3, label:"three"},
          {v:4, label:"four"}],
   "drawYAxis": false
};

function drawGraph() {
    var layout = new PlotKit.Layout("bar", options);
    layout.addDataset("sqrt", [[0, 0], [1, 1], [2, 1.414], [3, 1.73], [4, 2]]);
    layout.evaluate();
    var canvas = MochiKit.DOM.getElement("graph");
    var plotter = new PlotKit.SweetCanvasRenderer(canvas, layout, options);
    plotter.render();
}
MochiKit.DOM.addLoadEvent(drawGraph);

Here we define some additional options to affect how our graph is rendered.

1. First line defines where the IECanvasHTC behaviour file is so that we can have IE support.
2. Second line defines a new colorScheme to use. Here we are just using another preset color scheme that creates a palette out of the 6th preset base colour.
3. Third line defines some custom labels we would like by giving the mapping from X value to label.
4. Fourth line tells the renderer not to draw the Y axis.

Demonstration

To show you that is how it works, below is the graph defined exactly how it is presented in this quick start guide. On the left is a PNG of what you should expect and on the right is what it actually renders to.

Bar charts

Pie Charts

Author

Alastair Tse - Last Updated: 17th March 2006