Fix: Sometimes index === -1

Sometimes when mouse-overing container, i got `vendors.app.js:8427 Uncaught TypeError: Cannot read property 'xPos' of undefined` that is caused by `-1` index

src/js/charts/AxisChart.js

@@ -405,17 +405,19 @@ export default class AxisChart extends BaseChart {
 		if(!s.yExtremes) return;
 
 		let index = getClosestInArray(relX, s.xAxis.positions, true);
-		let dbi = this.dataByIndex[index];
-
-		this.tip.setValues(
-			dbi.xPos + this.tip.offset.x,
-			dbi.yExtreme + this.tip.offset.y,
-			{name: dbi.formattedLabel, value: ''},
-			dbi.values,
-			index
-		);
-
-		this.tip.showTip();
+		if (index >= 0) {
+			let dbi = this.dataByIndex[index];
+
+			this.tip.setValues(
+				dbi.xPos + this.tip.offset.x,
+				dbi.yExtreme + this.tip.offset.y,
+				{name: dbi.formattedLabel, value: ''},
+				dbi.values,
+				index
+			);
+
+			this.tip.showTip();
+		}
 	}
 
 	renderLegend() {