Provide an API using composition DP to build graphs

### Feature Proposal

Hi,

I wonder if it could be possible to provide an API using composition DP to build graphs in order to improve code extensibility, reuse, and readability.

For example :
```js
class MyCurve extends ??? {
attach(chart) {
chart.data.datasets.push({...});
// ... potentially changes other options.
}
detach() {
//...
}
}

mychart.addCurve( new MyCurve() ); // call MyCurve.attach(mychart)
mychart.addAxis( new AxisLabelWhatever('x', ['Toto', 'titi']) );
```

Indeed, in big graphs, configurations can quickly become a big chunk, hard to read.
Providing an API using composition DP would enable to better split big chunks of configurations, and facilitate code reusing/extensibility.

This would also enable to easily move curves (or other elements) from one graph to another, e.g. for a GUI graph editor or whatever.

### Possible Implementation

In a project I started (a PoC for an HTML binding of Chart.js), I use compositions DP with HTML elements :
https://github.com/denis-migdal/ChartsHTML

The JS code behind uses a composition DP, so this is a PoC for a possible implementation.