Add datetime support to _mean()

[m-ad]

This PR extends the _mean() function in shapeannotation.py to allow for sequences of datetime objects as input. This fixes issue #3065, specifically it enables annotations on hline/vline objects that use datetime axes.

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the code generator and not the generated files.
• I have added tests or modified existing tests.
• For a new feature, I have added documentation examples (please see the doc checklist as well).
• I have added a CHANGELOG entry if changing anything substantial.
• For a new feature or a change in behavior, I have updated the relevant docstrings in the code.

[gvwilson]

@emilykl please review

[emilykl]

@m-ad Thank you for the PR!

I'm open to something along these lines as a "patch" fix to enable add_vline() and add_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 that shape labels are natively supported in Plotly.js, I think the longer-term fix here is to refactor add_hline() and add_vline() to add annotation_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 returning x[0] is incorrect when x[0] and x[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 use add_hline() or add_vline() for a datetime axis you must pass the value as a datetime.

[robertclaus]

@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]

@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 to add_hline() and add_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.