Timelinize, just wow. Been wishing for something like this forever. Love the idea!

Regarding the gpx support... Don't want to lead you astray, but, have you seen https://www.sethoscope.net/heatmap/?

Thanks! Hopefully it delivers.

I haven't seen that, actually -- looks very nice. We do render a heatmap already, but I'm learning that heatmaps are very tricky to tune to be most useful/interesting. I will try to learn from that project.