fix(axis): fix handling x padding value

Make sure to handle correctly when axis.x.padding value is
given as number or as object.

Fix #2038

src/ChartInternal/Axis/Axis.ts

@@ -735,10 +735,11 @@ class Axis {
 	 * @returns {object} Padding object values with 'left' & 'right' key
 	 * @private
 	 */
-	getXAxisPadding(tickCount) {
+	getXAxisPadding(tickCount: number): {left: number, right: number} {
 		const $$ = this.owner;
-		let {left = 0, right = 0} = $$.config.axis_x_padding;
-		let padding = {left, right};
+		const padding = $$.config.axis_x_padding;
+		let {left = 0, right = 0} = isNumber(padding) ?
+			{left: padding, right: padding} : padding;
 
 		if ($$.axis.isTimeSeries()) {
 			const firstX = +$$.getXDomainMin($$.data.targets);
@@ -752,11 +753,9 @@ class Axis {
 				left = left / range / relativeTickWidth;
 				right = right / range / relativeTickWidth;
 			}
-
-			padding = {left, right};
 		}
 
-		return padding;
+		return {left, right};
 	}
 
 	updateLabels(withTransition) {

test/internals/axis-spec.ts

@@ -2509,6 +2509,59 @@ describe("AXIS", function() {
 
 			util.generate(option);			
 		});
+
+		it("set options", () => {
+			args = {
+				data: {
+					x: "x",
+					columns: [
+						["x", 1, 2, 3, 4, 5],
+						["data1", 30, 200, 100, 400, 150],
+					],
+					type: "line"
+				},
+				axis: {
+					x: {
+						type: "category",
+						tick: {
+							rotate: 5
+						},
+						padding: 10
+					}
+				}
+			}
+		});
+
+		it("check if axis.x.padding correctly set when is given as number value.", () => {
+			const {state} = chart.internal;
+			const padding = args.axis.x.padding;
+
+			expect(state.axis.x.padding).to.be.deep.equal({left: padding, right: padding});
+		});
+
+		it("set options axis.x.padding={left: 5}", () => {
+			args.axis.x.padding = {left: 5};
+		});
+
+		it("check if axis.x.padding correctly set when is given as 'left' key only.", () => {
+			const {state} = chart.internal;
+			const padding = args.axis.x.padding;
+
+			padding.right = 0;
+
+			expect(state.axis.x.padding).to.be.deep.equal(padding);
+		});
+
+		it("set options axis.x.padding={left: 15, right: 5}", () => {
+			args.axis.x.padding = {left: 15, right: 5};
+		});
+
+		it("check if axis.x.padding correctly set when is given as object type.", () => {
+			const {state} = chart.internal;
+			const padding = args.axis.x.padding;
+
+			expect(state.axis.x.padding).to.be.deep.equal(padding);
+		});
 	});
 
 	describe("axis min/max", () => {