Stacked scatter chart (#7468)

* Handle partially stacked scatter charts
* Update test files

src/core/core.datasetController.js

@@ -638,8 +638,9 @@ export default class DatasetController {
 		let i, value, parsed, otherValue;
 
 		function _compute() {
-			if (stack) {
-				stack.values = parsed._stacks[scale.axis];
+			const values = stack && parsed._stacks[scale.axis];
+			if (stack && values) {
+				stack.values = values;
 				// Need to consider individual stack values for data range,
 				// in addition to the stacked value
 				min = Math.min(min, value);

test/fixtures/controller.line/stacking/stacked-scatter.js

@@ -0,0 +1,68 @@
+module.exports = {
+	config: {
+		type: 'scatter',
+		data: {
+			datasets: [{
+				label: 'label1',
+				data: [{
+					x: 0,
+					y: 30
+				}, {
+					x: 5,
+					y: 35
+				}, {
+					x: 10,
+					y: 20
+				}],
+				backgroundColor: '#42A8E4'
+			},
+			{
+				label: 'label2',
+				data: [{
+					x: 0,
+					y: 10
+				}, {
+					x: 5,
+					y: 15
+				}, {
+					x: 10,
+					y: 15
+				}],
+				backgroundColor: '#FC3F55'
+			},
+			{
+				label: 'label3',
+				data: [{
+					x: 0,
+					y: -15
+				}, {
+					x: 5,
+					y: -10
+				}, {
+					x: 10,
+					y: -20
+				}],
+				backgroundColor: '#FFBE3F'
+			}],
+		},
+		options: {
+			scales: {
+				x: {
+					display: false,
+					position: 'bottom',
+				},
+				y: {
+					stacked: true,
+					display: false,
+					position: 'left',
+				},
+			},
+		},
+	},
+	options: {
+		canvas: {
+			height: 256,
+			width: 512
+		}
+	}
+};