swev-id: matplotlib__matplotlib-25960 honor subfigure spacing

[casey-brooks]

Summary

• implement subfigure bbox computation that honors explicit wspace/hspace in non-constrained layouts
• add geometry-based tests covering horizontal, vertical, default, and nested spacing
• ensure constrained layout path continues to rely on provided bboxes

Testing

• pytest lib/matplotlib/tests/test_figure.py -k 'subfigures_wspace_spacing or subfigures_hspace_spacing or subfigures_default_spacing_none or nested_subfigures_inherit_spacing'
• pytest lib/matplotlib/tests/test_figure.py -k 'subfigure_spanning or subfigure_tightbbox' --maxfail=1
• flake8 lib/matplotlib/figure.py lib/matplotlib/tests/test_figure.py

Issue

Fixes #64

Reproduction Steps

1. From matplotlib__matplotlib-25960, run pytest lib/matplotlib/tests/test_figure.py -k 'subfigures_wspace_spacing or subfigures_hspace_spacing or nested_subfigures_inherit_spacing'

Observed Failure (pre-fix)

============================= test session starts ==============================
platform linux -- Python 3.10.19, pytest-9.0.2, pluggy-1.6.0
rootdir: /workspace/matplotlib
configfile: pytest.ini
plugins: timeout-2.4.0, rerunfailures-16.1, xvfb-3.1.1, cov-7.0.0, xdist-3.8.0
collected 148 items / 144 deselected / 4 selected

lib/matplotlib/tests/test_figure.py FF.F                                 [100%]

=================================== FAILURES ===================================
________________________ test_subfigures_wspace_spacing ________________________

    def test_subfigures_wspace_spacing():
        fig = plt.figure()
        left, right = fig.subfigures(1, 2, wspace=0.2)
    
        expected_gap = 0.2 / (2 + 0.2 * (2 - 1))
        gap = right.bbox_relative.x0 - left.bbox_relative.x1
    
>       assert gap == pytest.approx(expected_gap)
E       assert 0.0 == 0.09090909090909091 ± 9.1e-08
E         
E         comparison failed
E         Obtained: 0.0
E         Expected: 0.09090909090909091 ± 9.1e-08

lib/matplotlib/tests/test_figure.py:312: AssertionError
________________________ test_subfigures_hspace_spacing ________________________

    def test_subfigures_hspace_spacing():
        fig = plt.figure()
        top, bottom = fig.subfigures(2, 1, hspace=0.3)
    
        expected_gap = 0.3 / (2 + 0.3 * (2 - 1))
        gap = top.bbox_relative.y0 - bottom.bbox_relative.y1
    
>       assert gap == pytest.approx(expected_gap)
E       assert 0.0 == 0.13043478260869565 ± 1.3e-07
E         
E         comparison failed
E         Obtained: 0.0
E         Expected: 0.13043478260869565 ± 1.3e-07

lib/matplotlib/tests/test_figure.py:324: AssertionError
____________________ test_nested_subfigures_inherit_spacing ____________________

    def test_nested_subfigures_inherit_spacing():
        fig = plt.figure()
        left, _ = fig.subfigures(1, 2, wspace=0.1)
        top, bottom = left.subfigures(2, 1, hspace=0.25)
    
        expected_gap = 0.25 / (2 + 0.25 * (2 - 1))
        gap = top.bbox_relative.y0 - bottom.bbox_relative.y1
    
>       assert gap == pytest.approx(expected_gap)
E       assert 0.0 == 0.1111111111111111 ± 1.1e-07
E         
E         comparison failed
E         Obtained: 0.0
E         Expected: 0.1111111111111111 ± 1.1e-07

lib/matplotlib/tests/test_figure.py:347: AssertionError
=========================== short test summary info ============================
FAILED lib/matplotlib/tests/test_figure.py::test_subfigures_wspace_spacing - ...
FAILED lib/matplotlib/tests/test_figure.py::test_subfigures_hspace_spacing - ...
FAILED lib/matplotlib/tests/test_figure.py::test_nested_subfigures_inherit_spacing
================= 3 failed, 1 passed, 144 deselected in 0.32s ==================

Fix

• SubFigure now delegates spacing-aware bbox calculation to GridSpec using a dummy SubplotParams so non-constrained wspace/hspace offsets create gaps while constrained layout keeps passing explicit bboxes.
• Added regression tests verifying non-constrained spacing works horizontally, vertically, and within nested subfigures, plus default spacing remains unchanged.

[casey-brooks]

Local Validation

• pytest lib/matplotlib/tests/test_figure.py -k 'subfigures_wspace_spacing or subfigures_hspace_spacing or subfigures_default_spacing_none or nested_subfigures_inherit_spacing' # 4 passed, 144 deselected
• pytest lib/matplotlib/tests/test_figure.py -k 'subfigure_spanning or subfigure_tightbbox' --maxfail=1 # 2 passed, 146 deselected
• flake8 lib/matplotlib/figure.py lib/matplotlib/tests/test_figure.py # no issues

[noa-lucent]

Solid fix. SubFigure now reuses GridSpec spacing logic so the manual wspace/hspace gaps materialize in the non-constrained path while bboxes provided by constrained layout stay untouched. New geometry assertions cover horizontal, vertical, default, and nested scenarios. Thanks!