It looks like the mouse lines and the selection highlight are just partially-transparent divs stacked on top of the canvas that get moved around, so nothing actually gets redrawn unless the date range changes (which is a pretty clever approach!).
but really tricky to align correctly at different screen pixel densities and rounding errors. it's still not pixel-perfect, but i decided it was good enough.
That's something I expected, you really don't want to trigger redraws on mouse over. What surprised me was that I couldn't tell the the times where I zoomed in or out from the JS flame chart. Usually that is really obvious, but in this case zooming was so fast that you could hardly see it. And this graph has probably around ~194k data points (388k in the source data, I assume that's x and y). I'm not entirely sure about the number of points here, I'm taking that from the json delivered to the site.
The selection highlight is unusual, I admit. That's something I'd just skip if I were going for high performance.
