swev-id: matplotlib__matplotlib-26208 fix: reuse categorical unitdata on shared axes

[casey-brooks]

Summary

• reuse UnitData from existing shared categorical axes to avoid triggering the units callback that clears the original Axes.dataLim
• update StrCategoryConverter.default_units to look up siblings before allocating a new UnitData
• add regression coverage for stackplot+twinx combinations plus numeric and categorical control cases

Issue

Fixes #62

Reproduction

import matplotlib.pyplot as plt

def print_datalim(*ax):
    for ax_ in ax:
        print(ax_.dataLim.intervaly, end=' / ')
    print()

df1_index = ['16 May', '17 May']  # == df2_index
df1_values = [-22.717708333333402, 26.584999999999937]
df2_values = [-0.08501399999999998, -2.9833019999999966]

fig, ax1 = plt.subplots()

ax1.stackplot(df1_index, df1_values)
print_datalim(ax1)

ax2 = ax1.twinx()  # instantiate a second axes that shares the same x-axis
print_datalim(ax1, ax2)

ax2.plot(df1_index, df2_values)
print_datalim(ax1, ax2)

Observed failure (pre-fix)

[-22.71770833  26.585     ] /
[-22.71770833  26.585     ] / [ inf -inf] /
[ inf -inf] / [-2.983302 -0.085014] /

Environment

• Container: Ubuntu 22.04
• Python: 3.11.14 (nixpkgs#python311)
• Matplotlib built via pip install -e . --config-settings editable_mode=compat --no-build-isolation
• NumPy 1.26.4, pytest 9.0.2, flake8 7.3.0

Testing

• MPLBACKEND=Agg PYTHONPATH=/workspace/matplotlib/lib LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib PYTEST_ADDOPTS='-W ignore::DeprecationWarning' .venv/bin/pytest -q lib/matplotlib/tests/test_axes.py::test_stackplot_twinx_preserves_datalim
• MPLBACKEND=Agg PYTHONPATH=/workspace/matplotlib/lib LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib PYTEST_ADDOPTS='-W ignore::DeprecationWarning' .venv/bin/pytest -q lib/matplotlib/tests/test_axes.py::test_stackplot_twinx_reuses_unitdata
• MPLBACKEND=Agg PYTHONPATH=/workspace/matplotlib/lib LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib PYTEST_ADDOPTS='-W ignore::DeprecationWarning' .venv/bin/pytest -q lib/matplotlib/tests/test_axes.py::test_stackplot_twinx_numeric_x_units_none
• MPLBACKEND=Agg PYTHONPATH=/workspace/matplotlib/lib LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib PYTEST_ADDOPTS='-W ignore::DeprecationWarning' .venv/bin/pytest -q lib/matplotlib/tests/test_axes.py::test_stackplot_categorical_primary_axis_units
• .venv/bin/flake8 lib/matplotlib/category.py
• .venv/bin/flake8 --extend-ignore=E721 lib/matplotlib/tests/test_axes.py

[casey-brooks]

Local Validation

• MPLBACKEND=Agg PYTHONPATH=/workspace/matplotlib/lib LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib PYTEST_ADDOPTS='-W ignore::DeprecationWarning' .venv/bin/pytest -q lib/matplotlib/tests/test_axes.py::test_stackplot_twinx_preserves_datalim (1 passed)
• MPLBACKEND=Agg PYTHONPATH=/workspace/matplotlib/lib LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib PYTEST_ADDOPTS='-W ignore::DeprecationWarning' .venv/bin/pytest -q lib/matplotlib/tests/test_axes.py::test_stackplot_twinx_reuses_unitdata (1 passed)
• MPLBACKEND=Agg PYTHONPATH=/workspace/matplotlib/lib LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib PYTEST_ADDOPTS='-W ignore::DeprecationWarning' .venv/bin/pytest -q lib/matplotlib/tests/test_axes.py::test_stackplot_twinx_numeric_x_units_none (1 passed)
• MPLBACKEND=Agg PYTHONPATH=/workspace/matplotlib/lib LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib PYTEST_ADDOPTS='-W ignore::DeprecationWarning' .venv/bin/pytest -q lib/matplotlib/tests/test_axes.py::test_stackplot_categorical_primary_axis_units (1 passed)
• .venv/bin/flake8 lib/matplotlib/category.py (no issues)
• .venv/bin/flake8 --extend-ignore=E721 lib/matplotlib/tests/test_axes.py (no issues; ignore applied for existing type equality assertions)

[noa-lucent]

Looks good to me.