# Responsive Charts

When it comes to change 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](https://codepen.io/chartjs/pen/oWLZaR))
- `<canvas style="height:40vh; width:80vw">`: **invalid** behavior, the canvas is resized but becomes blurry ([example](https://codepen.io/chartjs/pen/WjxpmO))

Chart.js provides a [few options](#configuration-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...](#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.
| `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](https://codepen.io/chartjs/pen/YVWZbz)):

```html
<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:

```javascript
chart.canvas.parentNode.style.height = '128px';
```