Add test_hlines to test_datetimes.py

[CozyFrog]

PR summary

I have added a datetime smoketest for Axes.hlines to lib/matplotlib/tests/test_datetime.py. This example is very similar to Axes.axhline (#27177) – but to test the entire functionality of hlines, I have done a combination of datetime.datetime/np.datetime64 arrays with both scalar and array xmin/xmax arguments.

Below is an image of the plot generated from this example code.

This addresses the Axes.hlines task from #26864.

PR checklist

• [N/A] "closes #0000" is in the body of the PR description to link the related issue
• [N/A] new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines

[ksunden]

I think we also wish to test the xmin/xmax as dates here

[CozyFrog]

Thank you for noting that, I did not think to check xmin/xmax as dates here - I have updated this test to do so, both with singular datetime values and datetime arrays.

This is what the plot generated by this example code looks like:

[CozyFrog]

I added singular float arguments for xmin/xmax - I think that might be why one of the CodeCov checks did not pass.

Here is what the new plot looks like:

[CozyFrog]

@ksunden @scottshambaugh could I please have some advice on why the codecov/project/tests check is not passing? It's at 98.76% "compared to a589856" - I don't know why it's comparing this to the test for scatter.

[CozyFrog]

If anybody is having the same issue, I think it stemmed simply from the fact that my branch was not current with the main body of work. Synchronizing it fixed the issue.