Bug summary

SubFigure with hspace=None incorrectly applies the default vertical spacing when wspace is specified. This produces an unintended vertical gap between subfigures.

Code for reproduction

import matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt

fig = plt.figure(figsize=(6, 4))
wspace, hspace = 0.2, None
print("Input: wspace=", wspace, "hspace=None")
subfigures = fig.subfigures(nrows=2, ncols=2,
                            wspace=wspace, hspace=hspace)

for row in range(2):
    for column in range(2):
        subfigures[row, column].add_subplot().plot([0, 1], [0, 1])

fig.canvas.draw()
for row in range(2):
    for column in range(2):
        print(f"Subfigure ({row}, {column}) bounds:",
              tuple(round(value, 3)
                    for value in subfigures[row, column].bbox.bounds))
plt.close(fig)

Actual outcome

Input: wspace= 0.2 hspace=None
Subfigure (0, 0) bounds: (np.float64(0.0), np.float64(218.182), np.float64(272.727), np.float64(181.818))
Subfigure (0, 1) bounds: (np.float64(327.273), np.float64(218.182), np.float64(272.727), np.float64(181.818))
Subfigure (1, 0) bounds: (np.float64(0.0), np.float64(0.0), np.float64(272.727), np.float64(181.818))
Subfigure (1, 1) bounds: (np.float64(327.273), np.float64(0.0), np.float64(272.727), np.float64(181.818))

Expected outcome

Input: wspace= 0.2 hspace=None
Subfigure (0, 0) bounds: (0.0, 200.0, 300.0, 200.0)
Subfigure (0, 1) bounds: (300.0, 200.0, 300.0, 200.0)
Subfigure (1, 0) bounds: (0.0, 0.0, 300.0, 200.0)
Subfigure (1, 1) bounds: (300.0, 0.0, 300.0, 200.0)

When no layout engine is active, hspace=None should result in no vertical spacing, while the requested wspace=0.2 should be applied.

Additional information

This is a regression introduced by PR #25960.

The example uses only public Matplotlib APIs and legal inputs, and its output is deterministic. The horizontal spacing is applied as requested, but hspace=None produces a vertical gap: the subfigure heights are 181.818 pixels with a 36.364-pixel gap. According to the updated documentation and the intent of PR #25960, unspecified spacing should be zero when no layout engine is active.

Passing hspace=None through GridSpec appears to fall back to the figure's default subplot hspace, causing unintended vertical spacing when only wspace is specified. The analogous issue may occur when only hspace is specified.

Operating system

Linux

Matplotlib Version

3.11.1

Matplotlib Backend

agg

Python version

3.11.13

Jupyter version

N/A

Installation

N/A