Try this for the line chart:

var originalLineController = Chart.controllers.line;
Chart.controllers.line = Chart.controllers.line.extend({
    draw: function() { 
        originalLineController.prototype.draw.apply(this, arguments);
        /* own drawing code here */

You can get the context with:

var ctx = this.chart.chart.ctx;


A bit late answer but you can extend the shape of the element, not the type of the specific chart, from v2.

For example,

Chart.elements.Rectangle.prototype.draw = function() {
   // Do your draw thingy.

There's an issue regarding this one at the official Github repo and also a great example of implementing it.

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