Will this break any existing users (aka is it opt-out instead of opt-in)?

Well the issue from #100 was asking for it to be opt-in, since it was added recently, right?

If matplotlib is installed, it's opt out. If matplotlib isn't installed, it doesn't run. So it should keep existing behavior.

So this should only break CI environments for users. That still is pretty bad, but I'm not sure there is a better solution.

No it shouldn't break ci for anyone. If matplotlib isn't installed it just turns off image generation

I wish there was a behavior where we automatically instead matplotlib and the user can explicitly choose not to instead it, instead of the opposite.

I don't think that's possible