Polar area: startAngle in degrees, 0 at top. (#6936)

* Polar area: startAngle in degrees, 0 at top.
Co-authored-by: Jukka Kurkela <jukka.kurkela@gmail.com>

docs/charts/polar.md

@@ -101,7 +101,7 @@ These are the customisation options specific to Polar Area charts. These options
 
 | Name | Type | Default | Description
 | ---- | ---- | ------- | -----------
-| `startAngle` | `number` | `-0.5 * Math.PI` | Starting angle to draw arcs for the first item in a dataset.
+| `startAngle` | `number` | `0` | Starting angle to draw arcs for the first item in a dataset. In degrees, 0 is at top.
 | `animation.animateRotate` | `boolean` | `true` | If true, the chart will animate in with a rotation animation. This property is in the `options.animation` object.
 | `animation.animateScale` | `boolean` | `true` | If true, will animate scaling the chart from the center outwards.
 
@@ -112,6 +112,7 @@ The polar area chart uses the [radialLinear](../axes/radial/linear.md) scale. Ad
 We can also change these defaults values for each PolarArea type that is created, this object is available at `Chart.defaults.polarArea`. Changing the global options only affects charts created after the change. Existing charts are not changed.
 
 For example, to configure all new polar area charts with `animateScale = false` you would do:
+
 ```javascript
 Chart.defaults.polarArea.animation.animateScale = false;
 ```

docs/getting-started/v3-migration.md

@@ -43,6 +43,7 @@ Chart.js 3.0 introduces a number of breaking changes. Chart.js 2.0 was released
 
 ### Options
 
+* `Polar area` `startAngle` option is now consistent with `Radar`, 0 is at top and value is in degrees. Default is changed from `-½π` to  `0`.
 * `scales.[x/y]Axes` arrays were removed. Scales are now configured directly to `options.scales` object with the object key being the scale Id.
 * `scales.[x/y]Axes.barPercentage` was moved to dataset option `barPercentage`
 * `scales.[x/y]Axes.barThickness` was moved to dataset option `barThickness`

src/controllers/controller.polarArea.js

@@ -32,7 +32,7 @@ defaults._set('polarArea', {
 		}
 	},
 
-	startAngle: -0.5 * Math.PI,
+	startAngle: 0,
 	legend: {
 		labels: {
 			generateLabels: function(chart) {
@@ -85,6 +85,12 @@ defaults._set('polarArea', {
 	}
 });
 
+function getStartAngleRadians(deg) {
+	// radialLinear scale draws angleLines using startAngle. 0 is expected to be at top.
+	// Here we adjust to standard unit circle used in drawing, where 0 is at right.
+	return helpers.math.toRadians(deg) - 0.5 * Math.PI;
+}
+
 export default DatasetController.extend({
 
 	dataElementType: elements.Arc,
@@ -117,13 +123,10 @@ export default DatasetController.extend({
 	},
 
 	update: function(mode) {
-		var me = this;
-		var meta = me._cachedMeta;
-		var arcs = meta.data;
+		const arcs = this._cachedMeta.data;
 
-		me._updateRadius();
-
-		me.updateElements(arcs, 0, mode);
+		this._updateRadius();
+		this.updateElements(arcs, 0, mode);
 	},
 
 	/**
@@ -154,7 +157,7 @@ export default DatasetController.extend({
 		const scale = chart.scales.r;
 		const centerX = scale.xCenter;
 		const centerY = scale.yCenter;
-		const datasetStartAngle = opts.startAngle || 0;
+		const datasetStartAngle = getStartAngleRadians(opts.startAngle);
 		let angle = datasetStartAngle;
 		let i;
 

src/helpers/helpers.math.js

@@ -83,11 +83,11 @@ export const sign = Math.sign ?
 	};
 
 export function toRadians(degrees) {
-	return degrees * (Math.PI / 180);
+	return degrees * (PI / 180);
 }
 
 export function toDegrees(radians) {
-	return radians * (180 / Math.PI);
+	return radians * (180 / PI);
 }
 
 /**
@@ -118,8 +118,8 @@ export function getAngleFromPoint(centrePoint, anglePoint) {
 
 	var angle = Math.atan2(distanceFromYCenter, distanceFromXCenter);
 
-	if (angle < (-0.5 * Math.PI)) {
-		angle += 2.0 * Math.PI; // make sure the returned angle is in the range of (-PI/2, 3PI/2]
+	if (angle < (-0.5 * PI)) {
+		angle += TAU; // make sure the returned angle is in the range of (-PI/2, 3PI/2]
 	}
 
 	return {

src/scales/scale.radialLinear.js

@@ -2,7 +2,7 @@
 
 import defaults from '../core/core.defaults';
 import helpers from '../helpers/index';
-import {isNumber, toDegrees} from '../helpers/helpers.math';
+import {isNumber, toDegrees, toRadians, _normalizeAngle} from '../helpers/helpers.math';
 import LinearScaleBase from './scale.linearbase';
 import Ticks from '../core/core.ticks';
 
@@ -157,7 +157,7 @@ function fitWithPointLabels(scale) {
 
 		// Add quarter circle to make degree 0 mean top of circle
 		var angleRadians = scale.getIndexAngle(i);
-		var angle = toDegrees(angleRadians) % 360;
+		var angle = toDegrees(angleRadians);
 		var hLimits = determineLimits(angle, pointPosition.x, textSize.w, 0, 180);
 		var vLimits = determineLimits(angle, pointPosition.y, textSize.h, 90, 270);
 
@@ -380,14 +380,11 @@ class RadialLinearScale extends LinearScaleBase {
 
 	getIndexAngle(index) {
 		var chart = this.chart;
-		var angleMultiplier = 360 / chart.data.labels.length;
+		var angleMultiplier = Math.PI * 2 / chart.data.labels.length;
 		var options = chart.options || {};
 		var startAngle = options.startAngle || 0;
 
-		// Start from the top instead of right, so remove a quarter of the circle
-		var angle = (index * angleMultiplier + startAngle) % 360;
-
-		return (angle < 0 ? angle + 360 : angle) * Math.PI * 2 / 360;
+		return _normalizeAngle(index * angleMultiplier + toRadians(startAngle));
 	}
 
 	getDistanceFromCenterForValue(value) {

test/fixtures/controller.polarArea/angle-lines.json

@@ -0,0 +1,35 @@
+{
+	"threshold": 0.05,
+    "config": {
+        "type": "polarArea",
+        "data": {
+            "labels": ["A", "B", "C", "D", "E"],
+            "datasets": [{
+                "data": [11, 16, 21, 7, 10],
+                "backgroundColor": [
+                    "rgba(255, 99, 132, 0.8)",
+                    "rgba(54, 162, 235, 0.8)",
+                    "rgba(255, 206, 86, 0.8)",
+                    "rgba(75, 192, 192, 0.8)",
+                    "rgba(153, 102, 255, 0.8)",
+                    "rgba(255, 159, 64, 0.8)"
+                ]
+            }]
+		},
+        "options": {
+            "responsive": false,
+            "legend": false,
+            "title": false,
+            "scale": {
+				"display": true,
+				"angleLines": {
+					"display": true,
+					"color": "#000"
+				},
+				"ticks": {
+					"display": false
+				}
+            }
+        }
+    }
+}

test/specs/controller.polarArea.tests.js

@@ -156,7 +156,7 @@ describe('Chart.controllers.polarArea', function() {
 				showLines: true,
 				legend: false,
 				title: false,
-				startAngle: 0, // default is -0.5 * Math.PI
+				startAngle: 90, // default is 0
 				elements: {
 					arc: {
 						backgroundColor: 'rgb(255, 0, 0)',

test/specs/scale.radialLinear.tests.js

@@ -535,7 +535,7 @@ describe('Test the radial linear scale', function() {
 			scale.ctx.getCalls().filter(function(x) {
 				return x.name === 'setTextAlign';
 			}).forEach(function(x, i) {
-				expect(x.args[0]).toBe(expected.textAlign[i]);
+				expect(x.args[0]).withContext('startAngle: ' + expected.startAngle + ', tick: ' + i).toBe(expected.textAlign[i]);
 			});
 
 			scale.ctx.getCalls().filter(function(x) {