Here's the Highcharts' stacked area chart demo:
https://www.highcharts.com/demo/area-stacked
If you click any of the data series in the legend at the bottom, it toggles that data series on and off in the chart itself.
From what I understand, Highcharts is what Splunk uses for charting, so it should be technically possible. Is this functionality supported in Splunk? This seems like something basic that every major charting library supports. But I looked everywhere in Splunk docs and haven't found anything.