Explicitly clear Matplotlib figure to remove warning about auto-removal of overlapping axes

[ianthomas23]

Running the Matplotlib tests currently gives a MatplotlibDeprecationWarning as follows:

$ pytest -v holoviews/tests/plotting/matplotlib/test_renderer.py::MPLRendererTest::test_get_size_row_plot
<snip>                                                                                                                    
holoviews/tests/plotting/matplotlib/test_renderer.py::MPLRendererTest::test_get_size_row_plot PASSED                               [100%]

============================================================ warnings summary ============================================================
holoviews/tests/plotting/matplotlib/test_renderer.py::MPLRendererTest::test_get_size_row_plot
  /Users/iant/github/holoviews/holoviews/plotting/mpl/plot.py:958: MatplotlibDeprecationWarning: Auto-removal of overlapping axes is deprecated since 3.6 and will be removed two minor releases later; explicitly call ax.remove() as needed.
    subaxes = [plt.subplot(self.gs[ind], projection=proj)

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
====================================================== 1 passed, 1 warning in 0.45s ======================================================

This PR explicitly clears the current Matplotlib figure using plt.clf(), which deletes all of its axes objects. The fix is added before the start of a loop that potentially calls plt.subplot multiple times as the clearing of the current figure only needs to occur the first time around.

With this fix all the Matplotlib tests pass locally without producing a warning. It does need to be checked by a HoloViews expert to confirm there won't be any unwanted side-effects.

[codecov-commenter]

Codecov Report

Merging #5857 (8d07a19) into main (75d510f) will increase coverage by 0.10%.
Report is 3 commits behind head on main.
The diff coverage is 88.67%.

@@            Coverage Diff             @@
##             main    #5857      +/-   ##
==========================================
+ Coverage   88.24%   88.34%   +0.10%     
==========================================
  Files         309      310       +1     
  Lines       63926    63927       +1     
==========================================
+ Hits        56412    56477      +65     
+ Misses       7514     7450      -64     

Flag	Coverage Δ
ui-tests	23.39% <26.41%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
holoviews/ipython/widgets.py	0.00% <0.00%> (ø)
holoviews/core/data/__init__.py	86.53% <92.85%> (ø)
holoviews/core/ndmapping.py	79.92% <100.00%> (+0.03%)	⬆️
holoviews/ipython/archive.py	44.79% <100.00%> (+5.66%)	⬆️
holoviews/ipython/magics.py	65.36% <100.00%> (+0.61%)	⬆️
holoviews/operation/downsample.py	84.90% <100.00%> (+84.90%)	⬆️
holoviews/plotting/bokeh/util.py	66.94% <100.00%> (+0.04%)	⬆️
holoviews/plotting/mpl/plot.py	60.74% <100.00%> (+0.05%)	⬆️
holoviews/plotting/plotly/renderer.py	56.25% <100.00%> (ø)
holoviews/tests/operation/test_downsample.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[philippjfr]

Thanks, makes sense to me!

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.