Describe the bug
It looks like it's partly a bug (vertical centering) and partly, a less than optimal user experience, or issue, depending on circumstances (horizontal snap with some offset, which causes the occlusion of the very hovered thing):

The cursor is not visible on the screenshot; it's over Deborah / 1965.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://elastic.github.io/elastic-charts/?path=/story/heatmap-alpha--categorical
2. Hover over cells, at various window widths

Expected behaviour
I think that ensuring that the tooltip box is always to the right and below (or maybe above) of the mouse pointer, or even the cell (in this case, with a callout arrow pointing to the cell) would be better

Checklist

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