Hovering over a heat map throws the error Uncaught TypeError: Cannot read property ‘getValueAxis’ of undefined

Seems like in these lines:

const aggId = d3.event.target.parentElement.__data__.aggId;
        const percentageMode = handler.charts[0].getSeries(aggId).getValueAxis().axisConfig.isPercentage();

aggId is undefined.

cc @elastic/kibana-visualizations

Only tried on master.