Make tooltip a little more robust

frontend/src/visualizations/components/ChartTooltip.jsx

@@ -25,7 +25,7 @@ export default class ChartTooltip extends Component {
         if (!hovered || !hovered.data || (pinToMouse ? !hovered.event : !hovered.element)) {
             return <span className="hidden" />;
         }
-        let s = series[hovered.seriesIndex || 0];
+        let s = series[hovered.seriesIndex] || series[0];
         return (
             <TooltipPopover
                 target={pinToMouse ? null : hovered.element}