Responsive Charts

Responsive Charts

When it comes to changing the chart size based on the window size, a major limitation is that the canvas render size (canvas.width and .height) can not be expressed with relative values, contrary to the display size (canvas.style.width and .height). Furthermore, these sizes are independent from each other and thus the canvas render size does not adjust automatically based on the display size, making the rendering inaccurate.

The following examples do not work:

<canvas height="40vh" width="80vw">: invalid values, the canvas doesn't resize (example)
<canvas style="height:40vh; width:80vw">: invalid behavior, the canvas is resized but becomes blurry (example)
Chart.js provides a few options to enable responsiveness and control the resize behavior of charts by detecting when the canvas display size changes and update the render size accordingly.

Configuration Options

Name	Type	Default	Description
`responsive`	`boolean`	`true`	Resizes the chart canvas when its container does (important note…).
`responsiveAnimationDuration`	`number`	`0`	Duration in milliseconds it takes to animate to new size after a resize event.
`maintainAspectRatio`	`boolean`	`true`	Maintain the original canvas aspect ratio `(width / height)` when resizing.
`aspectRatio`	`number`	`2`	Canvas aspect ratio (i.e. `width / height`, a value of 1 representing a square canvas). Note that this option is ignored if the height is explicitly defined either as attribute or via the style.
`onResize`	`function`	`null`	Called when a resize occurs. Gets passed two arguments: the chart instance and the new size.

Important Note

Detecting when the canvas size changes can not be done directly from the canvas element. Chart.js uses its parent container to update the canvas render and display sizes. However, this method requires the container to be relatively positioned and dedicated to the chart canvas only. Responsiveness can then be achieved by setting relative values for the container size (example):

<div class="chart-container" style="position: relative; height:40vh; width:80vw">
    <canvas id="chart"></canvas>
</div>

The chart can also be programmatically resized by modifying the container size:

chart.canvas.parentNode.style.height = '128px';
chart.canvas.parentNode.style.width = '128px';

Note that in order for the above code to correctly resize the chart height, the maintainAspectRatio option must also be set to false.

Printing Resizeable Charts

CSS media queries allow changing styles when printing a page. The CSS applied from these media queries may cause charts to need to resize. However, the resize won't happen automatically. To support resizing charts when printing, one needs to hook the onbeforeprint event and manually trigger resizing of each chart.

function beforePrintHandler () {
    for (var id in Chart.instances) {
        Chart.instances[id].resize();
    }
}