Chart.js/docs/developers/charts.md

# New Charts

Chart.js 2.0 introduced the concept of controllers for each dataset. Like scales, new controllers can be written as needed.

```javascript
class MyType extends Chart.DatasetController {

}

Chart.register(MyType);

// Now we can create a new instance of our chart, using the Chart.js API
new Chart(ctx, {
    // this is the string the constructor was registered at, ie Chart.controllers.MyType
    type: 'MyType',
    data: data,
    options: options
});
```

## Dataset Controller Interface

Dataset controllers must implement the following interface.

```javascript
{
    // Defaults for charts of this type
    defaults: {
        // If set to `false` or `null`, no dataset level element is created.
        // If set to a string, this is the type of element to create for the dataset.
        // For example, a line create needs to create a line element so this is the string 'line'
        datasetElementType: string | null | false,

        // If set to `false` or `null`, no elements are created for each data value.
        // If set to a string, this is the type of element to create for each data value.
        // For example, a line create needs to create a point element so this is the string 'point'
        dataElementType: string | null | false,
    }

    // ID of the controller
    id: string;

    // Update the elements in response to new data
    // @param mode : update mode, core calls this method using any of `'active'`, `'hide'`, `'reset'`, `'resize'`, `'show'` or `undefined`
    update: function(mode) {}
}
```

The following methods may optionally be overridden by derived dataset controllers.

```javascript
{
    // Draw the representation of the dataset. The base implementation works in most cases, and an example of a derived version
    // can be found in the line controller
    draw: function() {},

    // Initializes the controller
    initialize: function() {},

    // Ensures that the dataset represented by this controller is linked to a scale. Overridden to helpers.noop in the polar area and doughnut controllers as these
    // chart types using a single scale
    linkScales: function() {},

    // Parse the data into the controller meta data. The default implementation will work for cartesian parsing, but an example of an overridden
    // version can be found in the doughnut controller
    parse: function(start, count) {},
}
```

## Extending Existing Chart Types

Extending or replacing an existing controller type is easy. Simply replace the constructor for one of the built-in types with your own.

The built-in controller types are:

* `BarController`
* `BubbleController`
* `DoughnutController`
* `LineController`
* `PieController`
* `PolarAreaController`
* `RadarController`
* `ScatterController`

These controllers are also available in the UMD package, directly under `Chart`. Eg: `Chart.BarController`.

For example, to derive a new chart type that extends from a bubble chart, you would do the following.

```javascript
import {BubbleController} from 'chart.js';
class Custom extends BubbleController {
    draw() {
        // Call bubble controller method to draw all the points
        super.draw(arguments);

        // Now we can do some custom drawing for this dataset. Here we'll draw a red box around the first point in each dataset
        const meta = this.getMeta();
        const pt0 = meta.data[0];

        const {x, y} = pt0.getProps(['x', 'y']);
        const {radius} = pt0.options;

        const ctx = this.chart.ctx;
        ctx.save();
        ctx.strokeStyle = 'red';
        ctx.lineWidth = 1;
        ctx.strokeRect(x - radius, y - radius, 2 * radius, 2 * radius);
        ctx.restore();
    }
};
Custom.id = 'derivedBubble';
Custom.defaults = BubbleController.defaults;

// Stores the controller so that the chart initialization routine can look it up
Chart.register(Custom);

// Now we can create and use our new chart type
new Chart(ctx, {
    type: 'derivedBubble',
    data: data,
    options: options
});
```

## TypeScript Typings

If you want your new chart type to be statically typed, you must provide a `.d.ts` TypeScript declaration file. Chart.js provides a way to augment built-in types with user-defined ones, by using the concept of "declaration merging".

When adding a new chart type, `ChartTypeRegistry` must contain the declarations for the new type, either by extending an existing entry in `ChartTypeRegistry` or by creating a new one.

For example, to provide typings for a new chart type that extends from a bubble chart, you would add a `.d.ts` containing:

```typescript
import { ChartTypeRegistry } from 'chart.js';

declare module 'chart.js' {
    interface ChartTypeRegistry {
        derivedBubble: ChartTypeRegistry['bubble']
    }
}
```