To decrease the image diff I think it's good enough just add a separate test just checking png (as we don't expect to have anything that's backend-dependent here) and leave test_errorbar as it was initially?

Otherwise looks good to me.

To decrease the image diff I think it's good enough just add a separate test just checking png (as we don't expect to have anything that's backend-dependent here) and leave test_errorbar as it was initially?

👍 - I thought it was better to modify or add to an existing image than create a new one, but I guess that's not the case?

Ah right, I get your point about just adding a new .png test instead of changing all three images 👍

Thanks.
Now that the figure is bigger, it appears that the caps for theta-errorbars are drawn radially at the position of the cap, but perhaps it would make more sense to draw them to always be perpendicular to the errorbar instead? (radial caps would make sense if we drew the theta errorbars as circular arcs, but that's not the case currently)
(I guess it's "mostly" a matter of storing somewhere the info so that when you apply the rotation to the cap, you know where the center is.)

Hi, would you kindly consider changes in #20914 ? It will allow you specify transform during construction and not by overwriting _transform afterwards. However, it is not reviewed and I don’t know if it is a good practice to chain PRs.

Thanks for bringing that up - I've had a look at that PR, and will re-base this one if/when #20914 is merged.

I'll mark draft until #20914 gets in...

#20914 is in. Ping @dstansby ready for rebase.

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I'm going to close this, as I'm not enjoying trying to get my head round the logic in errorbar at the moment. I'll leave the branch though if anyone wants to pick this up and address #21006 (comment).

I take the liberty to reopen this and mark it as orphaned. May increase the chance of someone picking it up.

I'm going to close this in favour of #23592