title: Chart.js v3: Tooltip callback doesn't identify clicked dataset of stacked bar chart
body: <p>In Chart.js v2, the datasetIndex property of ChartTooltipItem[] identifies which segment of a stacked bar chart was clicked. This allowed the tooltip content to be customized for each segment of the stacked bar chart.</p>
  <p>In v3, TooltipItem[] provides the datasets but does not identify which one was clicked. There is a datasetIndex field for each TooltipItem, but it just matches the index in TooltipItem[] rather than identify the clicked segment.</p>
  <p>Has anyone found a field in the V3 tooltip callback to identified which segment of a stacked bar chart was clicked? Or was this functionality lost in the v3 rewrite?</p>