Johnny

@mejrs if this is not being worked on happy to take it on?

@iyakushev I have some local changes for a future PR, so yes working on it. Though some discussion on whether it should be: 1. A generic implementation? 2. If generic,...

Ok, thanks. I was thinking that `custom_hovertext` will apply to the whole heatmap, not just the core. I guess this is in essence `core_heatmap_hovertext`.

@imsnif is this still being worked on or relevant? Seems #112 was closed by mistake. @captain-yossarian I am happy to take this on if you are not working on it?