Stats: tooltip doesn't appear when moving cursor up on the insights posting calendar

[designsimply]

Steps to reproduce:

1. Go to https://wordpress.com/stats/insights and select a site if prompted
2. Find a column that has posts for each day in the column
3. Hover the mouse slowly down the column (tooltips work for each day)
4. Hover the mouse slowly up through the column (tooltips only work for every other day)

Result: the tooltips work normally when moving the mouse down, but not when it moves up.

I believe this is due to part of the tip component itself being overlayed on top of the insight day hover areas. So many hover targets so close together make this... fun.