Contour labelling image test failing against Matplotlib main

[rcomer]

Description

I just ran the tests against Matplotlib main, and got an image comparison failure because of the contour labels:

Expected

Actual

The test:

cartopy/lib/cartopy/tests/mpl/test_examples.py

Lines 33 to 72 in c4858e4

	@pytest.mark.natural_earth
	@pytest.mark.mpl_image_compare(filename='contour_label.png', tolerance=0.5)
	def test_contour_label():
	from cartopy.tests.mpl.test_caching import sample_data
	fig = plt.figure()
	# Setup a global EckertIII map with faint coastlines.
	ax = fig.add_subplot(1, 1, 1, projection=ccrs.EckertIII())
	ax.set_global()
	ax.coastlines('110m', alpha=0.1)
	# Use the waves example to provide some sample data, but make it
	# more dependent on y for more interesting contours.
	x, y, z = sample_data((20, 40))
	z = z * -1.5 * y
	# Add colourful filled contours.
	filled_c = ax.contourf(x, y, z, transform=ccrs.PlateCarree())
	# And black line contours.
	line_c = ax.contour(x, y, z, levels=filled_c.levels,
	colors=['black'],
	transform=ccrs.PlateCarree())
	# Uncomment to make the line contours invisible.
	# plt.setp(line_c.collections, visible=False)
	# Add a colorbar for the filled contour.
	fig.colorbar(filled_c, orientation='horizontal')
	# Use the line contours to place contour labels.
	ax.clabel(
	line_c, # Typically best results when labelling line contours.
	colors=['black'],
	manual=False, # Automatic placement vs manual placement.
	inline=True, # Cut the line where the label will be placed.
	fmt=' {:.0f} '.format, # Labes as integers, with some extra space.
	)
	return fig

I have two questions

1. Does anyone have a theory why inline=True does not seem to work any more? I cannot reproduce the problem with Matplotlib's Contour Label Demo.

2. How would you handle a genuine difference caused by different Matplotlib versions, so that both the main tests and the "minimum package versions" tests still pass?

Full environment definition

Operating system

RHEL7

Cartopy version

main

conda list

# Name                    Version                   Build  Channel
_libgcc_mutex             0.1                 conda_forge    conda-forge
_openmp_mutex             4.5                       2_gnu    conda-forge
alsa-lib                  1.2.8                h166bdaf_0    conda-forge
asttokens                 2.2.1              pyhd8ed1ab_0    conda-forge
atk-1.0                   2.38.0               hd4edc92_1    conda-forge
attr                      2.5.1                h166bdaf_1    conda-forge
attrs                     22.2.0             pyh71513ae_0    conda-forge
backcall                  0.2.0              pyh9f0ad1d_0    conda-forge
backports                 1.0                pyhd8ed1ab_3    conda-forge
backports.functools_lru_cache 1.6.4              pyhd8ed1ab_0    conda-forge
beautifulsoup4            4.12.2             pyha770c72_0    conda-forge
black                     23.3.0          py311h38be061_0    conda-forge
bleach                    6.0.0              pyhd8ed1ab_0    conda-forge
brotli                    1.0.9                h166bdaf_8    conda-forge
brotli-bin                1.0.9                h166bdaf_8    conda-forge
brotlipy                  0.7.0           py311hd4cff14_1005    conda-forge
bzip2                     1.0.8                h7f98852_4    conda-forge
c-ares                    1.18.1               h7f98852_0    conda-forge
ca-certificates           2022.12.7            ha878542_0    conda-forge
cairo                     1.16.0            h35add3b_1015    conda-forge
cairocffi                 1.4.0              pyhd8ed1ab_0    conda-forge
cartopy                   0.21.1.dev109+g08ee0828.d20230414          pypi_0    pypi
certifi                   2022.12.7          pyhd8ed1ab_0    conda-forge
cffi                      1.15.1          py311h409f033_3    conda-forge
cfgv                      3.3.1              pyhd8ed1ab_0    conda-forge
charset-normalizer        3.1.0              pyhd8ed1ab_0    conda-forge
click                     8.1.3           unix_pyhd8ed1ab_2    conda-forge
colorama                  0.4.6              pyhd8ed1ab_0    conda-forge
comm                      0.1.3              pyhd8ed1ab_0    conda-forge
contourpy                 1.0.7           py311ha3edf6b_0    conda-forge
coverage                  7.2.3           py311h2582759_0    conda-forge
cryptography              40.0.1          py311h9b4c7bb_0    conda-forge
cycler                    0.11.0             pyhd8ed1ab_0    conda-forge
cython                    0.29.34         py311hcafe171_0    conda-forge
dataclasses               0.8                pyhc8e2a94_3    conda-forge
dbus                      1.13.6               h5008d03_3    conda-forge
debugpy                   1.6.7           py311hcafe171_0    conda-forge
decorator                 5.1.1              pyhd8ed1ab_0    conda-forge
defusedxml                0.7.1              pyhd8ed1ab_0    conda-forge
distlib                   0.3.6              pyhd8ed1ab_0    conda-forge
entrypoints               0.4                pyhd8ed1ab_0    conda-forge
epoxy                     1.5.10               h166bdaf_1    conda-forge
exceptiongroup            1.1.1              pyhd8ed1ab_0    conda-forge
execnet                   1.9.0              pyhd8ed1ab_0    conda-forge
executing                 1.2.0              pyhd8ed1ab_0    conda-forge
expat                     2.5.0                hcb278e6_1    conda-forge
filelock                  3.11.0             pyhd8ed1ab_0    conda-forge
flake8                    6.0.0              pyhd8ed1ab_0    conda-forge
flake8-docstrings         1.7.0              pyhd8ed1ab_0    conda-forge
font-ttf-dejavu-sans-mono 2.37                 hab24e00_0    conda-forge
font-ttf-inconsolata      3.000                h77eed37_0    conda-forge
font-ttf-source-code-pro  2.038                h77eed37_0    conda-forge
font-ttf-ubuntu           0.83                 hab24e00_0    conda-forge
fontconfig                2.14.2               h14ed4e7_0    conda-forge
fonts-conda-ecosystem     1                             0    conda-forge
fonts-conda-forge         1                             0    conda-forge
fonttools                 4.39.3          py311h2582759_0    conda-forge
freetype                  2.12.1               hca18f0e_1    conda-forge
fribidi                   1.0.10               h36c2ea0_0    conda-forge
gdk-pixbuf                2.42.10              h6b639ba_2    conda-forge
geos                      3.11.2               hcb278e6_0    conda-forge
gettext                   0.21.1               h27087fc_0    conda-forge
glib                      2.74.1               h6239696_1    conda-forge
glib-tools                2.74.1               h6239696_1    conda-forge
graphite2                 1.3.13            h58526e2_1001    conda-forge
gst-plugins-base          1.22.0               h4243ec0_2    conda-forge
gstreamer                 1.22.0               h25f0c4b_2    conda-forge
gtk2                      2.24.33              h90689f9_2    conda-forge
gtk4                      4.10.1               hb50841d_0    conda-forge
harfbuzz                  6.0.0                h3ff4399_1    conda-forge
hicolor-icon-theme        0.17                 ha770c72_2    conda-forge
icu                       72.1                 hcb278e6_0    conda-forge
identify                  2.5.22             pyhd8ed1ab_0    conda-forge
idna                      3.4                pyhd8ed1ab_0    conda-forge
importlib-metadata        6.3.0              pyha770c72_0    conda-forge
importlib-resources       5.12.0             pyhd8ed1ab_0    conda-forge
importlib_metadata        6.3.0                hd8ed1ab_0    conda-forge
importlib_resources       5.12.0             pyhd8ed1ab_0    conda-forge
iniconfig                 2.0.0              pyhd8ed1ab_0    conda-forge
ipykernel                 6.22.0             pyh210e3f2_0    conda-forge
ipython                   8.12.0             pyh41d4057_0    conda-forge
jedi                      0.18.2             pyhd8ed1ab_0    conda-forge
jinja2                    3.1.2              pyhd8ed1ab_1    conda-forge
jsonschema                4.17.3             pyhd8ed1ab_0    conda-forge
jupyter_client            8.2.0              pyhd8ed1ab_0    conda-forge
jupyter_core              5.3.0           py311h38be061_0    conda-forge
jupyterlab_pygments       0.2.2              pyhd8ed1ab_0    conda-forge
keyutils                  1.6.1                h166bdaf_0    conda-forge
kiwisolver                1.4.4           py311h4dd048b_1    conda-forge
krb5                      1.20.1               h81ceb04_0    conda-forge
lame                      3.100             h166bdaf_1003    conda-forge
lcms2                     2.15                 haa2dc70_1    conda-forge
ld_impl_linux-64          2.40                 h41732ed_0    conda-forge
lerc                      4.0.0                h27087fc_0    conda-forge
libblas                   3.9.0           16_linux64_openblas    conda-forge
libbrotlicommon           1.0.9                h166bdaf_8    conda-forge
libbrotlidec              1.0.9                h166bdaf_8    conda-forge
libbrotlienc              1.0.9                h166bdaf_8    conda-forge
libcap                    2.67                 he9d0100_0    conda-forge
libcblas                  3.9.0           16_linux64_openblas    conda-forge
libclang                  16.0.1          default_h62803fd_0    conda-forge
libclang13                16.0.1          default_h9b593c0_0    conda-forge
libcups                   2.3.3                h36d4200_3    conda-forge
libcurl                   8.0.1                h588be90_0    conda-forge
libdeflate                1.18                 h0b41bf4_0    conda-forge
libedit                   3.1.20191231         he28a2e2_2    conda-forge
libev                     4.33                 h516909a_1    conda-forge
libevent                  2.1.10               h28343ad_4    conda-forge
libexpat                  2.5.0                hcb278e6_1    conda-forge
libffi                    3.4.2                h7f98852_5    conda-forge
libflac                   1.4.2                h27087fc_0    conda-forge
libgcc-ng                 12.2.0              h65d4601_19    conda-forge
libgcrypt                 1.10.1               h166bdaf_0    conda-forge
libgfortran-ng            12.2.0              h69a702a_19    conda-forge
libgfortran5              12.2.0              h337968e_19    conda-forge
libgirepository           1.74.0               ha2a38d2_1    conda-forge
libglib                   2.74.1               h606061b_1    conda-forge
libglu                    9.0.0             he1b5a44_1001    conda-forge
libgomp                   12.2.0              h65d4601_19    conda-forge
libgpg-error              1.46                 h620e276_0    conda-forge
libgraphene               1.10.6               h76c114f_0    conda-forge
libiconv                  1.17                 h166bdaf_0    conda-forge
libjpeg-turbo             2.1.5.1              h0b41bf4_0    conda-forge
liblapack                 3.9.0           16_linux64_openblas    conda-forge
libllvm16                 16.0.1               hadd5161_0    conda-forge
libnghttp2                1.52.0               h61bc06f_0    conda-forge
libnsl                    2.0.0                h7f98852_0    conda-forge
libogg                    1.3.4                h7f98852_1    conda-forge
libopenblas               0.3.21          pthreads_h78a6416_3    conda-forge
libopus                   1.3.1                h7f98852_1    conda-forge
libpng                    1.6.39               h753d276_0    conda-forge
libpq                     15.2                 hb675445_0    conda-forge
libsndfile                1.2.0                hb75c966_0    conda-forge
libsodium                 1.0.18               h36c2ea0_1    conda-forge
libsqlite                 3.40.0               h753d276_0    conda-forge
libssh2                   1.10.0               hf14f497_3    conda-forge
libstdcxx-ng              12.2.0              h46fd767_19    conda-forge
libsystemd0               253                  h8c4010b_1    conda-forge
libtiff                   4.5.0                ha587672_6    conda-forge
libuuid                   2.38.1               h0b41bf4_0    conda-forge
libvorbis                 1.3.7                h9c3ff4c_0    conda-forge
libwebp-base              1.3.0                h0b41bf4_0    conda-forge
libxcb                    1.13              h7f98852_1004    conda-forge
libxkbcommon              1.5.0                h79f4944_1    conda-forge
libxml2                   2.10.4               hfdac1af_0    conda-forge
libxslt                   1.1.37               h873f0b0_0    conda-forge
libzlib                   1.2.13               h166bdaf_4    conda-forge
lxml                      4.9.2           py311h14a6109_0    conda-forge
lz4-c                     1.9.4                hcb278e6_0    conda-forge
markupsafe                2.1.2           py311h2582759_0    conda-forge
matplotlib                3.8.0.dev1194+g26224d9606          pypi_0    pypi
matplotlib-inline         0.1.6              pyhd8ed1ab_0    conda-forge
mccabe                    0.7.0              pyhd8ed1ab_0    conda-forge
mistune                   2.0.5              pyhd8ed1ab_0    conda-forge
mpg123                    1.31.3               hcb278e6_0    conda-forge
munkres                   1.1.4              pyh9f0ad1d_0    conda-forge
mypy_extensions           1.0.0              pyha770c72_0    conda-forge
mysql-common              8.0.32               ha901b37_1    conda-forge
mysql-libs                8.0.32               hd7da12d_1    conda-forge
nbclient                  0.7.3              pyhd8ed1ab_0    conda-forge
nbconvert                 7.2.10             pyhd8ed1ab_1    conda-forge
nbconvert-core            7.2.10             pyhd8ed1ab_1    conda-forge
nbconvert-pandoc          7.2.10             pyhd8ed1ab_1    conda-forge
nbformat                  5.8.0              pyhd8ed1ab_0    conda-forge
ncurses                   6.3                  h27087fc_1    conda-forge
nest-asyncio              1.5.6              pyhd8ed1ab_0    conda-forge
nodeenv                   1.7.0              pyhd8ed1ab_0    conda-forge
nspr                      4.35                 h27087fc_0    conda-forge
nss                       3.89                 he45b914_0    conda-forge
numpy                     1.24.2          py311h8e6699e_0    conda-forge
openjpeg                  2.5.0                hfec8fc6_2    conda-forge
openssl                   3.1.0                h0b41bf4_0    conda-forge
owslib                    0.29.1             pyhd8ed1ab_0    conda-forge
packaging                 23.1               pyhd8ed1ab_0    conda-forge
pandas                    2.0.0           py311h2872171_0    conda-forge
pandoc                    2.19.2               h32600fe_2    conda-forge
pandocfilters             1.5.0              pyhd8ed1ab_0    conda-forge
pango                     1.50.14              hd33c08f_0    conda-forge
parso                     0.8.3              pyhd8ed1ab_0    conda-forge
pathlib2                  2.3.7.post1     py311h38be061_2    conda-forge
pathspec                  0.11.1             pyhd8ed1ab_0    conda-forge
pcre2                     10.40                hc3806b6_0    conda-forge
pexpect                   4.8.0              pyh1a96a4e_2    conda-forge
pickleshare               0.7.5                   py_1003    conda-forge
pillow                    9.5.0           py311h573f0d3_0    conda-forge
pip                       23.0.1             pyhd8ed1ab_0    conda-forge
pixman                    0.40.0               h36c2ea0_0    conda-forge
pkgutil-resolve-name      1.3.10             pyhd8ed1ab_0    conda-forge
platformdirs              3.2.0              pyhd8ed1ab_0    conda-forge
pluggy                    1.0.0              pyhd8ed1ab_5    conda-forge
ply                       3.11                       py_1    conda-forge
pooch                     1.7.0              pyha770c72_3    conda-forge
pre-commit                3.2.2              pyha770c72_0    conda-forge
proj                      9.2.0                h8ffa02c_0    conda-forge
prompt-toolkit            3.0.38             pyha770c72_0    conda-forge
prompt_toolkit            3.0.38               hd8ed1ab_0    conda-forge
psutil                    5.9.4           py311hd4cff14_0    conda-forge
pthread-stubs             0.4               h36c2ea0_1001    conda-forge
ptyprocess                0.7.0              pyhd3deb0d_0    conda-forge
pulseaudio-client         16.1                 h5195f5e_3    conda-forge
pure_eval                 0.2.2              pyhd8ed1ab_0    conda-forge
pybind11                  2.10.4          py311ha3edf6b_0    conda-forge
pybind11-global           2.10.4          py311ha3edf6b_0    conda-forge
pycairo                   1.23.0          py311hdece40b_0    conda-forge
pycodestyle               2.10.0             pyhd8ed1ab_0    conda-forge
pycparser                 2.21               pyhd8ed1ab_0    conda-forge
pydocstyle                6.3.0              pyhd8ed1ab_0    conda-forge
pyflakes                  3.0.1              pyhd8ed1ab_0    conda-forge
pygments                  2.15.0             pyhd8ed1ab_0    conda-forge
pygobject                 3.44.1          py311he2aa4b6_0    conda-forge
pykdtree                  1.3.7.post0     py311hcb2cf0a_0    conda-forge
pyopenssl                 23.1.1             pyhd8ed1ab_0    conda-forge
pyparsing                 3.0.9              pyhd8ed1ab_0    conda-forge
pyproj                    3.5.0           py311h1850bce_1    conda-forge
pyqt                      5.15.7          py311ha74522f_3    conda-forge
pyqt5-sip                 12.11.0         py311hcafe171_3    conda-forge
pyrsistent                0.19.3          py311h2582759_0    conda-forge
pyshp                     2.3.1              pyhd8ed1ab_0    conda-forge
pysocks                   1.7.1              pyha2e5f31_6    conda-forge
pytest                    7.3.0              pyhd8ed1ab_0    conda-forge
pytest-cov                4.0.0              pyhd8ed1ab_0    conda-forge
pytest-mpl                0.16.1             pyhd8ed1ab_0    conda-forge
pytest-rerunfailures      11.1.2             pyhd8ed1ab_0    conda-forge
pytest-timeout            2.1.0              pyhd8ed1ab_0    conda-forge
pytest-xdist              3.2.1              pyhd8ed1ab_0    conda-forge
python                    3.11.3          h2755cc3_0_cpython    conda-forge
python-dateutil           2.8.2              pyhd8ed1ab_0    conda-forge
python-fastjsonschema     2.16.3             pyhd8ed1ab_0    conda-forge
python-tzdata             2023.3             pyhd8ed1ab_0    conda-forge
python_abi                3.11                    3_cp311    conda-forge
pytz                      2023.3             pyhd8ed1ab_0    conda-forge
pyyaml                    6.0             py311hd4cff14_5    conda-forge
pyzmq                     25.0.2          py311hd6ccaeb_0    conda-forge
qt-main                   5.15.8               h5c52f38_9    conda-forge
readline                  8.2                  h8228510_1    conda-forge
requests                  2.28.2             pyhd8ed1ab_1    conda-forge
scipy                     1.10.1          py311h8e6699e_0    conda-forge
setuptools                67.6.1             pyhd8ed1ab_0    conda-forge
setuptools-scm            7.1.0              pyhd8ed1ab_0    conda-forge
setuptools_scm            7.1.0                hd8ed1ab_0    conda-forge
shapely                   2.0.1           py311h54d622a_1    conda-forge
sip                       6.7.8           py311hcafe171_0    conda-forge
six                       1.16.0             pyh6c4a22f_0    conda-forge
snowballstemmer           2.2.0              pyhd8ed1ab_0    conda-forge
soupsieve                 2.3.2.post1        pyhd8ed1ab_0    conda-forge
sqlite                    3.40.0               h4ff8645_0    conda-forge
stack_data                0.6.2              pyhd8ed1ab_0    conda-forge
tinycss2                  1.2.1              pyhd8ed1ab_0    conda-forge
tk                        8.6.12               h27826a3_0    conda-forge
toml                      0.10.2             pyhd8ed1ab_0    conda-forge
tomli                     2.0.1              pyhd8ed1ab_0    conda-forge
tornado                   6.2             py311hd4cff14_1    conda-forge
traitlets                 5.9.0              pyhd8ed1ab_0    conda-forge
typing-extensions         4.5.0                hd8ed1ab_0    conda-forge
typing_extensions         4.5.0              pyha770c72_0    conda-forge
tzdata                    2023c                h71feb2d_0    conda-forge
ukkonen                   1.0.1           py311h4dd048b_3    conda-forge
urllib3                   1.26.15            pyhd8ed1ab_0    conda-forge
virtualenv                20.21.0            pyhd8ed1ab_0    conda-forge
wcwidth                   0.2.6              pyhd8ed1ab_0    conda-forge
webencodings              0.5.1                      py_1    conda-forge
wheel                     0.40.0             pyhd8ed1ab_0    conda-forge
wxpython                  4.2.0           py311h0cf6651_6    conda-forge
xcb-util                  0.4.0                h166bdaf_0    conda-forge
xcb-util-image            0.4.0                h166bdaf_0    conda-forge
xcb-util-keysyms          0.4.0                h166bdaf_0    conda-forge
xcb-util-renderutil       0.3.9                h166bdaf_0    conda-forge
xcb-util-wm               0.4.1                h166bdaf_0    conda-forge
xkeyboard-config          2.38                 h0b41bf4_0    conda-forge
xorg-compositeproto       0.4.2             h7f98852_1001    conda-forge
xorg-damageproto          1.2.1             h7f98852_1002    conda-forge
xorg-fixesproto           5.0               h7f98852_1002    conda-forge
xorg-inputproto           2.3.2             h7f98852_1002    conda-forge
xorg-kbproto              1.0.7             h7f98852_1002    conda-forge
xorg-libice               1.0.10               h7f98852_0    conda-forge
xorg-libsm                1.2.3             hd9c2040_1000    conda-forge
xorg-libx11               1.8.4                h0b41bf4_0    conda-forge
xorg-libxau               1.0.9                h7f98852_0    conda-forge
xorg-libxcomposite        0.4.6                h0b41bf4_1    conda-forge
xorg-libxcursor           1.2.0                h0b41bf4_1    conda-forge
xorg-libxdamage           1.1.5                h7f98852_1    conda-forge
xorg-libxdmcp             1.1.3                h7f98852_0    conda-forge
xorg-libxext              1.3.4                h0b41bf4_2    conda-forge
xorg-libxfixes            5.0.3             h7f98852_1004    conda-forge
xorg-libxi                1.7.10               h7f98852_0    conda-forge
xorg-libxinerama          1.1.5                h27087fc_0    conda-forge
xorg-libxrandr            1.5.2                h7f98852_1    conda-forge
xorg-libxrender           0.9.10            h7f98852_1003    conda-forge
xorg-randrproto           1.5.0             h7f98852_1001    conda-forge
xorg-renderproto          0.11.1            h7f98852_1002    conda-forge
xorg-util-macros          1.19.3               h7f98852_0    conda-forge
xorg-xextproto            7.3.0             h0b41bf4_1003    conda-forge
xorg-xf86vidmodeproto     2.3.1             h7f98852_1002    conda-forge
xorg-xineramaproto        1.2.1             h7f98852_1001    conda-forge
xorg-xproto               7.0.31            h7f98852_1007    conda-forge
xz                        5.2.6                h166bdaf_0    conda-forge
yaml                      0.2.5                h7f98852_2    conda-forge
zeromq                    4.3.4                h9c3ff4c_1    conda-forge
zipp                      3.15.0             pyhd8ed1ab_0    conda-forge
zlib                      1.2.13               h166bdaf_4    conda-forge
zstd                      1.5.2                h3eb15da_6    conda-forge

[rcomer]

The behaviour change comes from matplotlib/matplotlib#25247.

[rcomer]

Just realised that @greglucas already has a plan for this: matplotlib/matplotlib#25247 (comment)

[greglucas]

Just realised that @greglucas already has a plan for this: matplotlib/matplotlib#25247 (comment)

I can't say I actually had a plan for this ;) just that I looked at the code within Cartopy and noticed that there was a lot of path manipulation with the clabel subclass, so at first glance it seems like we should address it here rather than in MPL. I wonder if we can get rid of the GeoContourSet altogether now that it is a single PathCollection in MPL?

[rcomer]

So I tried this to just not use GeoContourSet: rcomer@f86924d

and got this. All the other contour tests pass with this commit and that straight dashed line at ~45 north only appears with the call to clabel.

I also tried naively changing the GeoContourSet.clabel method so it applies its procedure to self's paths rather than each PathCollection's, but that was way worse. I guess it breaks the ContourSet's knowledge of which paths correspond to which level.

[greglucas]

That is interesting. That first case looks really close to what we want. My initial guess of what is happening here is that we have a contour calculated in data-space. Then that specific one is on the wrapping boundary, so then it gets cut and we try to resplice the paths within the path interpolation code, but then the text gets inserted and messes up that line interpolation portion... That is a wild guess though and I'm not sure how easy it would be to fix that either.

[rcomer]

I have a sketch of a solution at matplotlib/matplotlib#26297. That, combined with my Cartopy commit mentioned above (to not use GeoContourSet) gives us

Or with a smaller font (so we get more labels):