Added 'devicePixelRatio' option to override the window's DPR setting

docs/general/README.md

@@ -5,4 +5,5 @@ These sections describe general configuration options that can apply elsewhere i
 * [Colors](./colors.md) defines acceptable color values
 * [Font](./fonts.md) defines various font options
 * [Interactions](./interactions/README.md) defines options that reflect how hovering chart elements works
-* [Responsive](./responsive.md) defines responsive chart options that apply to all charts.
+* [Responsive](./responsive.md) defines responsive chart options that apply to all charts.
+* [Device Pixel Ratio](./device-pixel-ratio.md) defines the ratio between display pixels and rendered pixels 

docs/general/device-pixel-ratio.md

@@ -0,0 +1,13 @@
+# Device Pixel Ratio
+
+By default the chart's canvas will use a 1:1 pixel ratio, unless the physical display has a higher pixel ratio (e.g. Retina displays).
+
+For applications where a chart will be converted to a bitmap, or printed to a higher DPI medium it can be desirable to render the chart at a higher resolution than the default.
+
+Setting `devicePixelRatio` to a value other than 1 will force the canvas size to be scaled by that amount, relative to the container size. There should be no visible difference on screen; the difference will only be visible when the image is zoomed or printed.
+
+## Configuration Options
+
+| Name | Type | Default | Description
+| ---- | ---- | ------- | -----------
+| `devicePixelRatio` | `Number` | window.devicePixelRatio | Override the window's default devicePixelRatio.

src/core/core.controller.js

@@ -130,7 +130,7 @@ module.exports = function(Chart) {
 			// Before init plugin notification
 			plugins.notify(me, 'beforeInit');
 
-			helpers.retinaScale(me);
+			helpers.retinaScale(me, me.options.devicePixelRatio);
 
 			me.bindEvents();
 
@@ -181,7 +181,7 @@ module.exports = function(Chart) {
 			canvas.style.width = newWidth + 'px';
 			canvas.style.height = newHeight + 'px';
 
-			helpers.retinaScale(me);
+			helpers.retinaScale(me, options.devicePixelRatio);
 
 			if (!silent) {
 				// Notify any plugins about the resize

src/core/core.helpers.js

@@ -816,8 +816,8 @@ module.exports = function(Chart) {
 			el.currentStyle[property] :
 			document.defaultView.getComputedStyle(el, null).getPropertyValue(property);
 	};
-	helpers.retinaScale = function(chart) {
-		var pixelRatio = chart.currentDevicePixelRatio = window.devicePixelRatio || 1;
+	helpers.retinaScale = function(chart, forceRatio) {
+		var pixelRatio = chart.currentDevicePixelRatio = forceRatio || window.devicePixelRatio || 1;
 		if (pixelRatio === 1) {
 			return;
 		}

test/jasmine.matchers.js

@@ -122,7 +122,7 @@ function toBeChartOfSize() {
 			var message = null;
 			var canvas = actual.ctx.canvas;
 			var style = getComputedStyle(canvas);
-			var pixelRatio = window.devicePixelRatio;
+			var pixelRatio = actual.options.devicePixelRatio || window.devicePixelRatio;
 			var dh = parseInt(style.height, 10);
 			var dw = parseInt(style.width, 10);
 			var rh = canvas.height;

test/specs/core.controller.tests.js

@@ -555,6 +555,55 @@ describe('Chart', function() {
 		});
 	});
 
+	describe('config.options.devicePixelRatio 3', function() {
+		beforeEach(function() {
+			this.devicePixelRatio = window.devicePixelRatio;
+			window.devicePixelRatio = 1;
+		});
+
+		afterEach(function() {
+			window.devicePixelRatio = this.devicePixelRatio;
+		});
+
+		// see https://github.com/chartjs/Chart.js/issues/3575
+		it ('should scale the render size but not the "implicit" display size', function() {
+			var chart = acquireChart({
+				options: {
+					responsive: false,
+					devicePixelRatio: 3
+				}
+			}, {
+				canvas: {
+					width: 320,
+					height: 240,
+				}
+			});
+
+			expect(chart).toBeChartOfSize({
+				dw: 320, dh: 240,
+				rw: 960, rh: 720,
+			});
+		});
+
+		it ('should scale the render size but not the "explicit" display size', function() {
+			var chart = acquireChart({
+				options: {
+					responsive: false,
+					devicePixelRatio: 3
+				}
+			}, {
+				canvas: {
+					style: 'width: 320px; height: 240px'
+				}
+			});
+
+			expect(chart).toBeChartOfSize({
+				dw: 320, dh: 240,
+				rw: 960, rh: 720,
+			});
+		});
+	});
+
 	describe('controller.destroy', function() {
 		it('should remove the resizer element when responsive: true', function() {
 			var chart = acquireChart({