I can't flag the type error cause it's not in this PR, but I get the feeling it's gonna complicate things here b/c it implies you only need to pass one.

I'm sorry I'm not quite following this

The error message that checks the parameters says that exactly one must be given. I think given is ambiguous enough in this context that one valid reading is that only one parameter must be passed in.

@story645 thanks I’m with you now (my head was stuck in mypy type errors). The majority of users who see that error will be using ax.axline, so this error makes sense for them. Not sure what the best thing to do here is.

I might be missing a beat, why can't both parameters be defaulted to none in the init?

I came here to say the same as @story645.

(With that said, I may also be missing something here...)

However, not sure that such a change should be backported? And an API-change note is required.

(Also, one may consider documenting that **kwargs are forwarded to Line2D.)

Fully agree that requiring xy2 and slope is not a good API. Ideally, we would have introduced a class method from_slope tor the latter. Though not sure that it's worth changing now.

I'm fine with introducing None-defaults.