@emilykl please review

@m-ad Thank you for the PR!

I'm open to something along these lines as a "patch" fix to enableadd_vline() andadd_hline() to work with datetime axes, although as@nicolaskruchtenmentioned, the "correct" fix would be to refactor the code such that we're not trying to do math with data values.

Ultimately, now thatshape labels are natively supported in Plotly.js, I think the longer-term fix here is to refactoradd_hline() andadd_vline() to addannotation_text as a shape label rather than as a separate annotation; that will allow us to skip the step of computing averages of the data values. I mention it here just in case that's a task you'd be interested in -- I've opened issue#5373 to track.

Otherwise, something like what you've proposed here will work as a temporary fix.

There's a few issues with the logic as written; for example, falling back to returningx[0] is incorrect whenx[0] andx[1] are not equal. We should simply raise an error in that case because we have no way of calculating an average.

That means cases like the reproducible example in#3065, where the date value is passed as a string, will require a different approach. I'm not sure exactly what the best approach is; trying to parse every value as a datetime seems a bit clunky, but technically speaking it will produce the correct result. Or maybe we just document this limitation and specify that if you are trying to useadd_hline() oradd_vline() for a datetime axis you must pass the value as a datetime.

@m-ad since we haven't seen any activity on this PR in a while, we may close it in the near future. Let us know if you are still working on it!

@m-ad since we haven't seen any activity on this PR in a while, we may close it in the near future. Let us know if you are still working on it!

@robertclaus I am not currently working on it. This is a is a band-aid-style fix and as@emilykl said, the nicer and more "correct" version would be to implement the changes toadd_hline() andadd_vline() proposed in#5373. Someone appears to be working on that, which would ideally make this PR unnecessary. That said, if you still like to have my band-aid fix around until#5373 is closed, I'd be happy to finalize this PR.