suspected mpl 3.4.1 regression for a graphical test

[bjlittle]

🐛 Bug Report

How To Reproduce

As part of #4087, the perceptual image hash images have been updated to account for the associated changes within iris from the recent release of matplotlib 3.4.1.

As part of #4087, SciTools/test-iris-imagehash#39 was raised to update the test-iris-imagehash archive.

Consequently, there appears to be a regression for one the graphical test iris.tests.test_mapping.TestLimitedAreaCube.test_grid.0, see here for further details.

Steps to reproduce the behaviour:

From the root directory of iris:

1. git checkout 7fd7960e25cefc48c49a6e518cfd02f42fa860ea
2. pip install --no-deps --editable .
3. python setup.py test
4. python lib/iris/tests/idiff.py --results iris_image_test_output

Environment

• Iris Version: as per the iris commit SHA
• Matplotlib Version: 3.4.1

Additional context

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[bjlittle]

@SciTools/peloton An ideal issues for the peloton to chase down 👍

[rcomer]

The comment on that test suggests that the grid is supposed to be blue and the colours were caused by a matplotlib bug. 😕

iris/lib/iris/tests/test_mapping.py

Lines 241 to 244 in 83309c3

	def test_grid(self):
	iplt.pcolormesh(self.cube, facecolors="none", edgecolors="blue")
	# the result is a graphic which has coloured edges. This is a mpl bug,
	# see https://github.com/matplotlib/matplotlib/issues/1302

[rcomer]

Maybe the fact that this test result has changed is telling us we can now remove the extra handling in iris.plot.outline 🤔

iris/lib/iris/plot.py

Lines 1276 to 1280 in 83309c3

	# set the _is_stroked property to get a single color grid.
	# See https://github.com/matplotlib/matplotlib/issues/1302
	result._is_stroked = False
	if hasattr(result, "_wrapped_collection_fix"):
	result._wrapped_collection_fix._is_stroked = False

[rcomer]

History: #109