CategoryScale: automatically add missing labels

src/scales/scale.category.js

@@ -1,5 +1,14 @@
 import Scale from '../core/core.scale';
 
+function findOrAddLabel(labels, raw, index) {
+	const first = labels.indexOf(raw);
+	if (first === -1) {
+		return typeof raw === 'string' ? labels.push(raw) - 1 : index;
+	}
+	const last = labels.lastIndexOf(raw);
+	return first !== last ? index : first;
+}
+
 export default class CategoryScale extends Scale {
 
 	constructor(cfg) {
@@ -12,12 +21,8 @@ export default class CategoryScale extends Scale {
 
 	parse(raw, index) {
 		const labels = this.getLabels();
-		if (labels[index] === raw) {
-			return index;
-		}
-		const first = labels.indexOf(raw);
-		const last = labels.lastIndexOf(raw);
-		return first === -1 || first !== last ? index : first;
+		return isFinite(index) && labels[index] === raw
+			? index : findOrAddLabel(labels, raw, index);
 	}
 
 	determineDataLimits() {

test/fixtures/controller.bar/bar-skip-null-object-data.js

@@ -2,7 +2,6 @@ module.exports = {
 	config: {
 		type: 'bar',
 		data: {
-			labels: [0, 1, 3, 4],
 			datasets: [
 				{
 					data: {0: 5, 1: 20, 2: 1, 3: 10},

test/specs/scale.category.tests.js

@@ -90,6 +90,30 @@ describe('Category scale tests', function() {
 		expect(getLabels(scale)).toEqual(labels);
 	});
 
+	it('Should generate missing labels', function() {
+		var labels = ['a', 'b', 'c', 'd'];
+		var chart = window.acquireChart({
+			type: 'line',
+			data: {
+				datasets: [{
+					data: {a: 1, b: 3, c: -1, d: 10}
+				}]
+			},
+			options: {
+				scales: {
+					x: {
+						type: 'category',
+						labels: ['a']
+					}
+				}
+			}
+		});
+
+		var scale = chart.scales.x;
+		expect(getLabels(scale)).toEqual(labels);
+
+	});
+
 	it('should get the correct label for the index', function() {
 		var chart = window.acquireChart({
 			type: 'line',