[Feature] Tooltip missing trigger: stack

### What problem does this feature solve?

Context:
A very basic chart showing bars with 5 (or N) series (activities), 2 (or N) stacks (people) , and several categories (weeks).
There is a jsfiddle https://jsfiddle.net/j6e19myk/1/ showing a working example.

All works fine thanks to your excellent work.

Feature:
All works fine, but the tooltip. I'd like a kind of option like "trigger: stack" so the tooltip could display only the values belonging to the stack being under the mouse.
Unless my data should be rearranged but I can't imagine how, since the results are as expected.

Best regards
Pierre.

### What does the proposed API look like?

This is only a feature request.

Start with the linked JSFiddle and inspect the current tooltip behavior for the multi-series, multi-stack bar chart. Determine how a stack is identified under the pointer and define what a `trigger: stack` option should display; the work is done when the hovered stack shows only its corresponding values without changing existing tooltip behavior.

Written by the indexing model from the issue text.