Fix add_vline text annotation with dates

[jjrob13]

Background

I would like to use add_vline with a text annotation.

Closes: #3065

The workaround presented in #3065 (comment) breaks if people view the plot in a difference timezone than where it was generated.

As mentioned in that ticket, here is the error when data is used for text annotations TypeError: unsupported operand type(s) for +: 'int' and 'str'

The backtrace looks like:

> /lib/python3.9/site-packages/plotly/basedatatypes.py(4085)add_vline()
-> self._process_multiple_axis_spanning_shapes(
  /lib/python3.9/site-packages/plotly/basedatatypes.py(4031)_process_multiple_axis_spanning_shapes()
-> augmented_annotation = shapeannotation.axis_spanning_shape_annotation(
  /lib/python3.9/site-packages/plotly/shapeannotation.py(216)axis_spanning_shape_annotation()
-> shape_dict = annotation_params_for_line(
  /lib/python3.9/site-packages/plotly/shapeannotation.py(63)annotation_params_for_line()
-> eX = _mean(X)
  /lib/python3.9/site-packages/plotly/shapeannotation.py(7)_mean()
-> return float(sum(x)) / len(x)

For line annotations, the same x value is passed for both x1/x2, so the mean is trivial to compute.

4084        ):
4085 ->         self._process_multiple_axis_spanning_shapes(
4086                dict(type="line", x0=x, x1=x, y0=0, y1=1),
4087                row,
4088                col,
4089                "vline",
4090                exclude_empty_subplots=exclude_empty_subplots,

-> return float(sum(x)) / len(x)
(Pdb) p len(x)
2
(Pdb) p x[0] == x[1]
True

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 codegen files and not generated files.
• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
• For a new feature, I have added documentation examples in an existing or
  new tutorial notebook (please see the doc checklist as well).
• I have added a CHANGELOG entry if fixing/changing/adding anything substantial.
• For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).

Test Plan

I'm now able to use add_vline and text annotations with dates. The repro in #3065 works now

[jjrob13]

cc: @nicolaskruchten this is a small bugfix that is useful for us, and it looks like people have been reporting it a few times (see PR description for issue)

[gvwilson]

@marthacryan can you please have a look at this one (and if possible add a test when merging) thanks - @gvwilson