Remove matplotlib 3.8+ pin

[kafitzgerald]

Describe the bug
The plot_contour_labels usage example is not working properly with matplotlib 3.8+.

Most of this was because of issues with matplotlib and cartopy many of which have now been addressed, but few label locations are impacted by matplotlib/matplotlib#27333. Particularly for this usage example I think we should still just remove this pin and thin the number of labels (including the problematic ones).

You can see the problems here:

• https://geocat-viz.readthedocs.io/en/latest/examples/plot_contour_labels.html (spurious lines added with the contour labels, which also look a bit off)
• This also caused a CI failure over on geocat-examples for NCL_coneff_8.py (fails with an error about indexing in the updated matplotlib code - seems to be somewhat dependent upon the location of the labels)

IndexError                                Traceback (most recent call last)
Cell In[12], line 41
     35 manual = [(-70, 55000), (-80, 26000), (-72, 22500), (-62, 40000), (-58, 30000),
     36           (-45, 69500), (-40, 34000), (-12, 39000), (-37, 75000), (40, 50000),
     37           (-25, 42000), (18, 23000), (30, 40000), (45, 40000), (57, 41000),
     38           (63, 39000), (55, 80000), (65, 85000)]
     40 manual = [(-70, 55000), (-80, 26000), (55, 80000), (65, 85000)]
---> 41 clabels = ax1.clabel(contours,
     42                      levels=np.arange(0, 25, 8),
     43                      fontsize=12,
     44                      colors="black",
     45                      fmt="%.0f",
     46                      manual=manual)
     47 plt.show()

File ~/miniconda3/envs/geocat/lib/python3.11/site-packages/matplotlib/axes/_axes.py:6550, in Axes.clabel(self, CS, levels, **kwargs)
   6532 def clabel(self, CS, levels=None, **kwargs):
   6533     """
   6534     Label a contour plot.
   6535 
   (...)
   6548         All other parameters are documented in `~.ContourLabeler.clabel`.
   6549     """
-> 6550     return CS.clabel(levels, **kwargs)

File ~/miniconda3/envs/geocat/lib/python3.11/site-packages/matplotlib/contour.py:210, in ContourLabeler.clabel(self, levels, fontsize, inline, inline_spacing, fmt, colors, use_clabeltext, manual, rightside_up, zorder)
    208 if np.iterable(manual):
    209     for x, y in manual:
--> 210         self.add_label_near(x, y, inline, inline_spacing)
    211 elif manual:
    212     print('Select label locations manually using first mouse button.')

File ~/miniconda3/envs/geocat/lib/python3.11/site-packages/matplotlib/contour.py:590, in ContourLabeler.add_label_near(self, x, y, inline, inline_spacing, transform)
    587 label_width = self._get_nth_label_width(level)
    588 rotation, path = self._split_path_and_get_label_rotation(
    589     path, idx_vtx_min, proj, label_width, inline_spacing)
--> 590 self.add_label(*proj, rotation, self.labelLevelList[idx_level_min],
    591                self.labelCValueList[idx_level_min])
    593 if inline:
    594     self._paths[idx_level_min] = path

IndexError: list index out of range

Likely related to these changes:

• https://matplotlib.org/stable/api/prev_api_changes/api_changes_3.8.0.html#contourset-is-now-a-single-collection
• Turn ContourSet into a standard Collection artist. matplotlib/matplotlib#25247

Related issues over on cartopy:

• Contour labelling image test failing against Matplotlib main SciTools/cartopy#2207
• contourf issues with mpl3.8 SciTools/cartopy#2224

Where I'm at in sorting this out:

• Have tested with the latest matplotlib and cartopy and am seeing the problems (fine w/ latest matplotlib 3.7)
• Messed around a bit with the plot_contour_labels example and it seems only some label locations trigger the issue (generally those closest to the edge of the contour / projected space)
• Similarly the NCL_coneff_8.py example on fails for certain labels
• Unfortunately sorting this out will likely require a deeper dive in matplotlib / cartopy than I have time for at the moment

I think the should throw in a version pin for the time being.

Side note:

• We could also check out what cartopy is doing for testing.

[kafitzgerald]

Actually, I think this may be a bug in matplotlib. Something doesn't seem to be working properly in the code that finds the nearby contours to label.

Unfortunately we don't get an error in the plot_contour_labels usage example like we do in geocat-examples, but the usage example is also impacted. See here: https://geocat-viz.readthedocs.io/en/latest/examples/plot_contour_labels.html

I think we should probably pin to <3.8 and I'll report something over on the matplotlib repo once I have a little better of an idea what's going on.

[kafitzgerald]

Ok, it looks like this is more than one issue though some have been resolved by fixes upstream.

The remaining (after updating matplotlib and cartopy) spurious lines in the plot_contour_labels usage example are occurring because of one remaining bug in matplotlib. In particular, these three points are problematic:

(-58.78, 67.05)
(-102.69, 73.62)
(-78.02, 52.22)

The remaining labels still overlap a bit more than they used to, but I think we should probably just thin out the number of labeled contour locations for the usage example.