[Bug] wspace/hspace ignored in Figure.subfigures (ID: 281)

[rowan-stein]

User Request

[Bug]: wspace and hspace in subfigures not working

Bug summary:

• wspace and hspace in Figure.subfigures do nothing.

Code for reproduction:

import matplotlib.pyplot as plt

figs = plt.figure().subfigures(2, 2, wspace=0, hspace=0)
for fig in figs.flat:
    fig.subplots().plot([1, 2])
plt.show()

Actual outcome:

• Same figure independently of the values of hspace and wspace.

Expected outcome:

• Spacing should be reflected, consistent with Figure.subfigures behavior (see upstream reference in figure.py around lines 1550-1554).

Environment (from reporter):

• OS/X, Matplotlib 3.7.1, Backend MacOSX, Python 3.10.9, install via conda.

Research Specification (from Emerson Gray)

Summary:

• In non-constrained layout, SubFigure placement ignores GridSpec’s wspace/hspace. The fix is to compute SubFigure bbox in parent coordinates applying provided wspace and hspace from Figure.subfigures.

Key code paths:

• lib/matplotlib/figure.py:
  • Figure.subfigures(...) creates a GridSpec and SubFigures.
  • SubFigure._redo_transform_rel_fig(bbox=None) computes subfigure region but currently ignores spacing unless constrained layout passes an explicit bbox.
• lib/matplotlib/gridspec.py: GridSpec.get_grid_positions(...) shows the reference logic for spacing.
• lib/matplotlib/_constrained_layout.py: constrained layout path already handles spacing by calling _redo_transform_rel_fig with a bbox.

Algorithm (non-constrained):

• When _redo_transform_rel_fig is called with bbox=None, compute the subfigure’s rectangle in the parent coordinate system [0,1] using nrows/ncols, width/height ratios, and wspace/hspace from the GridSpec created by Figure.subfigures.
• Treat wspace/hspace as 0.0 when None (backward-compatible default; previously there were no gaps).
• Do not involve figure.subplotpars margins; subfigures fill the parent area.
• For a SubplotSpec span (e.g., gs[r0:r1, c0:c1]), use min left/max right and min bottom/max top over spanned cells including internal separators.

Edge cases:

• (wspace,hspace) = (0,0): subfigures touch.
• nrows=1 or ncols=1: spacing has no effect (full size).
• Nested subfigures: compute inside the parent’s [0,1] space.
• Mixed subplots and subfigures: leave constrained behavior unchanged; non-constrained defaults remain zero spacing if None.

Testing plan:

• Add geometry-based tests checking that wspace/hspace produce measurable gaps without constrained layout.
• Test that default None yields no gaps.
• Test nested subfigures behavior.

Implementation changes:

• Modify SubFigure._redo_transform_rel_fig(self, bbox=None) in lib/matplotlib/figure.py to compute bbox_relative using spacing per the algorithm above when bbox is None. Keep the bbox is not None branch unchanged.