Chart.js

//Global Chart helpers object for utility methods and classes
var helpers = Chart.helpers = {};

//-- Basic js utility methods
var each = helpers.each = function(loopable,callback,self){
	
	// [...]
	
	getConstraintWidth = helpers.getConstraintWidth = function(domNode){ // returns Number or undefined if no constraint

		var constrainedWidth,
			constrainedWNode      = document.defaultView.getComputedStyle(domNode)['max-width'],
			constrainedWContainer = document.defaultView.getComputedStyle(domNode.parentNode)['max-width'],
			hasCWNode      = constrainedWNode !== null && constrainedWNode !== "none",
			hasCWContainer = constrainedWContainer !== null && constrainedWContainer !== "none";

		if(hasCWNode || hasCWContainer){

			constrainedWidth = Math.min((hasCWNode      ? parseInt(constrainedWNode, 10)      : Number.POSITIVE_INFINITY),
										(hasCWContainer ? parseInt(constrainedWContainer, 10) : Number.POSITIVE_INFINITY));
		}
		return constrainedWidth;
	},
	getConstraintHeight = helpers.getConstraintHeight = function(domNode){ // returns Number or undefined if no constraint

		var constrainedHeight,
			constrainedHNode      = document.defaultView.getComputedStyle(domNode)['max-height'],
			constrainedHContainer = document.defaultView.getComputedStyle(domNode.parentNode)['max-height'],
			hasCHNode      = constrainedHNode !== null && constrainedHNode !== "none",
			hasCHContainer = constrainedHContainer !== null && constrainedHContainer !== "none";

		if(constrainedHNode || constrainedHContainer){

			constrainedHeight = Math.min((hasCHNode      ? parseInt(constrainedHNode, 10)      : Number.POSITIVE_INFINITY),
										 (hasCHContainer ? parseInt(constrainedHContainer, 10) : Number.POSITIVE_INFINITY));
		}
		return constrainedHeight;
	},
	getMaximumWidth = helpers.getMaximumWidth = function(domNode){
		var container = domNode.parentNode;
		// TODO = check cross browser stuff with this.
		var w = container.clientWidth;
		var cw = getConstraintWidth(domNode);
		if(cw !== undefined) w = Math.min(w, cw);
		
		return w;
	},
	getMaximumHeight = helpers.getMaximumHeight = function(domNode){
		var container = domNode.parentNode;
		// TODO = check cross browser stuff with this.
		var h = container.clientHeight;
		var ch = getConstraintHeight(domNode);
		if(ch !== undefined) h = Math.min(h, ch);

		return h;
	},
	
// [...]

//Core methods that'll be a part of every chart type
extend(Chart.Type.prototype,{
	
	// [...] 
	resize : function(callback){
		this.stop();
		var canvas = this.chart.canvas,
			newWidth = getMaximumWidth(this.chart.canvas),
			newHeight = this.options.maintainAspectRatio ? newWidth / this.chart.aspectRatio : getMaximumHeight(this.chart.canvas);

		// if aspect ratio must be respected we gotta explicitly
		// check if we don't break the height constraint, if this
		// is the case we recompute the width
		if(this.options.maintainAspectRatio){
			
			var ch = getConstraintHeight(this.chart.canvas);
			if(ch !== undefined && ch < newHeight){

				newHeight = ch;
				newWidth = newHeight * this.chart.aspectRatio;
			}
		}

		canvas.width = this.chart.width = newWidth;
		canvas.height = this.chart.height = newHeight;

		retinaScale(this.chart);

		if (typeof callback === "function"){
			callback.apply(this, Array.prototype.slice.call(arguments, 1));
		}
		return this;
	},