Nice. Would you be interested in adding tests to help us 🔒 down this feature?

Any recommendation about where to add these tests? Is there a pattern somewhere for testing hovers?

OK, I added tests for scatterpolar and scatterpolargl because the test harness was set up for it. I tried to replicate the test harness for barpolar and scatterternary but I wasn't able to trick assertHoverLabelContent into finding anything :(

I tried to replicate the test harness for barpolar and scatterternary but I wasn't able to trick

Right, because barpolar and scatterternary hover test don't actually test Fx.hover (where the template substitutions happen). They instead test their trace module's .hoverPoints method - which only feeds in pointData.hovertemplate to Fx.hover.

So, an ok test for both barpolar and scatterternary would be to assert that pointData.hovertemplate is defined after hoverPoints().

OK, done :)

Nice PR! 💃 - thanks for your help @nicolaskruchten !

Happy to help! I might submit a followup for scattergeo and scattermapbox if they're also using the easy case.

I might submit a followup for scattergeo and scattermapbox

Yeah, those should be straight-forward and similar to this PR.

Trace types scatter3d, surface, mesh3d and parcats might be more of a pain (but doable), their implementation would be similar to pie in #3126

so nice to see it automagically work in RCE :)