add check on overwriting canvas height/width (chartjs#4874)

* add check on overwriting canvas height/width

* unit test for this

src/core/core.helpers.js

@@ -512,8 +512,10 @@ module.exports = function(Chart) {
 		// If no style has been set on the canvas, the render size is used as display size,
 		// making the chart visually bigger, so let's enforce it to the "correct" values.
 		// See https://github.com/chartjs/Chart.js/issues/3575
-		canvas.style.height = height + 'px';
-		canvas.style.width = width + 'px';
+		if (!canvas.style.height && !canvas.style.width) {
+			canvas.style.height = height + 'px';
+			canvas.style.width = width + 'px';
+		}
 	};
 	// -- Canvas methods
 	helpers.fontString = function(pixelSize, fontStyle, fontFamily) {

test/specs/core.helpers.tests.js

@@ -725,6 +725,23 @@ describe('Core helper tests', function() {
 		document.body.removeChild(div);
 	});
 
+	it ('should leave styled height and width on canvas if explicitly set', function() {
+		var chart = window.acquireChart({}, {
+			canvas: {
+				height: 200,
+				width: 200,
+				style: 'height: 400px; width: 400px;'
+			}
+		});
+
+		helpers.retinaScale(chart, true);
+
+		var canvas = chart.canvas;
+
+		expect(canvas.style.height).toBe('400px');
+		expect(canvas.style.width).toBe('400px');
+	});
+
 	describe('Color helper', function() {
 		function isColorInstance(obj) {
 			return typeof obj === 'object' && obj.hasOwnProperty('values') && obj.values.hasOwnProperty('rgb');