Fix waterfall 'closest' hover when cursor is below the size axis

[etpinard]

fixes #4524 - essentially a followup on #3778

before: https://codepen.io/etpinard/pen/dyPBwPJ
after: https://codepen.io/etpinard/pen/povXqJv

cc @archmoj

[archmoj]

@etpinard thanks for the PR!
Any idea why the hover does not show up on the very first bar?

[etpinard]

Try now: https://codepen.io/etpinard/pen/oNgrJYq

[archmoj]

Nicely done.
💃

[etpinard]

Ok thanks for the review. I'll wait before you have a look at the treemap_coffee problem before merging.