CanvasRenderer = function(el, options) {
	var cachedBackground;
	var canvas = document.createElement('canvas');

	el.appendChild(canvas);

	if (typeof(G_vmlCanvasManager) !== 'undefined') {
		G_vmlCanvasManager.initElement(canvas);
	}

	var ctx = canvas.getContext('2d');

	canvas.width = canvas.height = options.size;

	// canvas on retina devices
	var scaleBy = 1;
	if (window.devicePixelRatio > 1) {
		scaleBy = window.devicePixelRatio;
		canvas.style.width = canvas.style.height = [options.size, 'px'].join('');
		canvas.width = canvas.height = options.size * scaleBy;
		ctx.scale(scaleBy, scaleBy);
	}

	// move 0,0 coordinates to the center
	ctx.translate(options.size / 2, options.size / 2);

	// rotate canvas -90deg
	ctx.rotate((-1 / 2 + options.rotate / 180) * Math.PI);

	var radius = (options.size - options.lineWidth) / 2;
	if (options.scaleColor && options.scaleLength) {
		radius -= options.scaleLength + 2; // 2 is the distance between scale and bar
	}

	// IE polyfill for Date
	Date.now = Date.now || function() {
		return +(new Date());
	};

	/**
	 * Draw a circle around the center of the canvas
	 * @param {strong} color     Valid CSS color string
	 * @param {number} lineWidth Width of the line in px
	 * @param {number} percent   Percentage to draw (float between -1 and 1)
	 */
	var drawCircle = function(color, lineWidth, percent) {
		percent = Math.min(Math.max(-1, percent || 0), 1);
		var isNegative = percent <= 0 ? true : false;

		ctx.beginPath();
		ctx.arc(0, 0, radius, 0, Math.PI * 2 * percent, isNegative);

		ctx.strokeStyle = color;
		ctx.lineWidth = lineWidth;

		ctx.stroke();
	};

	/**
	 * Draw the scale of the chart
	 */
	var drawScale = function() {
		var offset;
		var length;

		ctx.lineWidth = 1;
		ctx.fillStyle = options.scaleColor;

		ctx.save();
		for (var i = 24; i > 0; --i) {
			if (i % 6 === 0) {
				length = options.scaleLength;
				offset = 0;
			} else {
				length = options.scaleLength * 0.6;
				offset = options.scaleLength - length;
			}
			ctx.fillRect(-options.size/2 + offset, 0, length, 1);
			ctx.rotate(Math.PI / 12);
		}
		ctx.restore();
	};

	/**
	 * Request animation frame wrapper with polyfill
	 * @return {function} Request animation frame method or timeout fallback
	 */
	var reqAnimationFrame = (function() {
		return  window.requestAnimationFrame ||
				window.webkitRequestAnimationFrame ||
				window.mozRequestAnimationFrame ||
				function(callback) {
					window.setTimeout(callback, 1000 / 60);
				};
	}());

	/**
	 * Draw the background of the plugin including the scale and the track
	 */
	var drawBackground = function() {
		if(options.scaleColor) drawScale();
		if(options.trackColor) drawCircle(options.trackColor, options.trackWidth || options.lineWidth, 1);
	};

  /**
    * Canvas accessor
   */
  this.getCanvas = function() {
    return canvas;
  };

  /**
    * Canvas 2D context 'ctx' accessor
   */
  this.getCtx = function() {
    return ctx;
  };

	/**
	 * Clear the complete canvas
	 */
	this.clear = function() {
		ctx.clearRect(options.size / -2, options.size / -2, options.size, options.size);
	};

	/**
	 * Draw the complete chart
	 * @param {number} percent Percent shown by the chart between -100 and 100
	 */
	this.draw = function(percent) {
		// do we need to render a background
		if (!!options.scaleColor || !!options.trackColor) {
			// getImageData and putImageData are supported
			if (ctx.getImageData && ctx.putImageData) {
				if (!cachedBackground) {
					drawBackground();
					cachedBackground = ctx.getImageData(0, 0, options.size * scaleBy, options.size * scaleBy);
				} else {
					ctx.putImageData(cachedBackground, 0, 0);
				}
			} else {
				this.clear();
				drawBackground();
			}
		} else {
			this.clear();
		}

		ctx.lineCap = options.lineCap;

		// if barcolor is a function execute it and pass the percent as a value
		var color;
		if (typeof(options.barColor) === 'function') {
			color = options.barColor(percent);
		} else {
			color = options.barColor;
		}

		// draw bar
		drawCircle(color, options.lineWidth, percent / 100);
	}.bind(this);

	/**
	 * Animate from some percent to some other percentage
	 * @param {number} from Starting percentage
	 * @param {number} to   Final percentage
	 */
	this.animate = function(from, to) {
		var startTime = Date.now();
		options.onStart(from, to);
		var animation = function() {
			var process = Math.min(Date.now() - startTime, options.animate.duration);
			var currentValue = options.easing(this, process, from, to - from, options.animate.duration);
			this.draw(currentValue);
			options.onStep(from, to, currentValue);
			if (process >= options.animate.duration) {
				options.onStop(from, to);
			} else {
				reqAnimationFrame(animation);
			}
		}.bind(this);

		reqAnimationFrame(animation);
	}.bind(this);
}
Renderer to render the chart on a canvas object
@param {DOMElement} el      DOM element to host the canvas (root of the plugin)
@param {object}     options options object of the plugin