Contours not displaying properly with matplotlib 3.8

[ktyle]

In an environment running an up-to-date Holoviz set of packages:

geoviews = 1.10.1
holoviews = 1.17.1
bokeh = 3.2.2
panel = 1.2.3

I’ve discovered that both line and filled contours do not properly display if Matplotlib 3.8.0 is installed. Things work properly with MPL 3.7.3.

Here is a fully-reproducible code block:

import geoviews as gv
import xarray as xr

gv.extension("bokeh")

Z = xr.open_dataset(
    "http://thredds.atmos.albany.edu:8080/thredds/dodsC/atm533/2012103000_z500_era5.nc"
).z

gv.LineContours(Z.isel(time=0), kdims=["longitude", "latitude"], vdims=["z"])

Here are a couple of screenshots (with a background map layer) showing the expected (pre-MPL 3.8) and problematic (MPL 3.8.0) visualizations:

Good (MPL 3.7.3):

Problematic (MPL 3.8.0):

[ahuang11]

Copying my thoughts from https://discourse.holoviz.org/t/contours-not-displaying-properly-with-matplotlib-3-8/6068/4

I suppose it's a HoloViews issue actually: https://holoviews.org/_modules/holoviews/operation/element.html#contours

    def _process(self, element, key=None):
        try:
            from matplotlib.contour import QuadContourSet
            from matplotlib.axes import Axes
            from matplotlib.figure import Figure
            from matplotlib.dates import num2date, date2num
        except ImportError:
            raise ImportError("contours operation requires matplotlib.")
        extent = element.range(0) + element.range(1)[::-1]

Or perhaps a matplotlib issue since we're just using QuadContourSet?

Or because of this:
https://matplotlib.org/stable/api/prev_api_changes/api_changes_3.8.0.html#contourset-is-now-a-single-collection

And we need to loop differently:

Prior to this release, ContourSet (the object returned by contour) was a custom object holding multiple Collections (and not an Artist) -- one collection per level, each connected component of that level's contour being an entry in the corresponding collection.

ContourSet is now instead a plain Collection (and thus an Artist). The collection contains a single path per contour level; this path may be non-continuous in case there are multiple connected components.

[github-actions]

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