Make sure bezier control points are updated (chartjs#7710)

* Make sure bezier control points are updated
* Test

Author: kurkle

src/controllers/controller.line.js

@@ -103,6 +103,11 @@ export default class LineController extends DatasetController {
 		const lastPoint = data[data.length - 1].size();
 		return Math.max(border, firstPoint, lastPoint) / 2;
 	}
+
+	draw() {
+		this._cachedMeta.dataset.updateControlPoints(this.chart.chartArea);
+		super.draw();
+	}
 }
 
 LineController.id = 'line';

src/elements/element.line.js

@@ -194,6 +194,7 @@ export default class Line extends Element {
 		this._fullLoop = undefined;
 		this._points = undefined;
 		this._segments = undefined;
+		this._pointsUpdated = false;
 
 		if (cfg) {
 			Object.assign(this, cfg);
@@ -203,9 +204,10 @@ export default class Line extends Element {
 	updateControlPoints(chartArea) {
 		const me = this;
 		const options = me.options;
-		if (options.tension && !options.stepped) {
+		if (options.tension && !options.stepped && !me._pointsUpdated) {
 			const loop = options.spanGaps ? me._loop : me._fullLoop;
 			_updateBezierControlPoints(me._points, options, chartArea, loop);
+			me._pointsUpdated = true;
 		}
 	}
 
@@ -338,6 +340,8 @@ export default class Line extends Element {
 
 		ctx.stroke();
 		ctx.restore();
+
+		this._pointsUpdated = false;
 	}
 }
 

test/specs/controller.line.tests.js

@@ -79,6 +79,7 @@ describe('Chart.controllers.line', function() {
 		});
 
 		var meta = chart.getDatasetMeta(0);
+		spyOn(meta.dataset, 'updateControlPoints');
 		spyOn(meta.dataset, 'draw');
 		spyOn(meta.data[0], 'draw');
 		spyOn(meta.data[1], 'draw');
@@ -87,6 +88,8 @@ describe('Chart.controllers.line', function() {
 
 		chart.update();
 
+		expect(meta.dataset.updateControlPoints.calls.count()).toBeGreaterThanOrEqual(1);
+		expect(meta.dataset.draw.calls.count()).toBe(1);
 		expect(meta.data[0].draw.calls.count()).toBe(1);
 		expect(meta.data[1].draw.calls.count()).toBe(1);
 		expect(meta.data[2].draw.calls.count()).toBe(1);