Stack chart hovering shouldn't redraw the entire screen

We only need to draw 2 boxes when moving and changing the highlighted stack item, the previously highlighted one, and the next highlighted one. We shouldn't redraw the entire chart in such a case. This will help things feel snappier, especially since we're already doing a decent amount of work with rendering the tooltip.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-728)