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Fix #21409: Make twin axes inherit parent position #31353

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Fix #21409: Make twin axes inherit parent position #31353

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d9ef7af
Fix #21409: Make twin axes inherit parent position
maf2310 Mar 23, 2026
88d4ec5
Refine twin position fix to preserve layout participation
maf2310 Mar 24, 2026
a5435b5
Address review: preserve original/active positions and fix test + lint
maf2310 Mar 25, 2026
ce749b4
Clarify manual-position twin sync in code comment
maf2310 Mar 25, 2026
5876d06
Add regression test for twinx top spine position
maf2310 Mar 27, 2026
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14 changes: 14 additions & 0 deletions lib/matplotlib/axes/_base.py
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Original file line number Diff line number Diff line change
Expand Up @@ -4657,6 +4657,20 @@ def _make_twin_axes(self, *args, **kwargs):
twin.set_zorder(self.zorder)

self._twinned_axes.join(self, twin)

# If the parent Axes has been manually positioned (set_position() sets
# in_layout=False), the SubplotSpec-based add_subplot(...) path ignores
# that manual position when creating a twin. In that case, explicitly
# copy both the original and active positions to the twin so they start
# aligned.
#
# For layout-managed Axes (in_layout=True), we keep the existing
# SubplotSpec-driven behavior, so layout engines such as tight_layout
# and constrained_layout continue to control positioning.
if not self.get_in_layout():
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@jklymak jklymak Mar 27, 2026

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I think this is all fine, but needs to be tested.

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@maf2310 maf2310 Mar 27, 2026

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Thanks! I’ll add additional tests to cover this case.

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@maf2310 maf2310 Mar 27, 2026

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I added tests to cover both manual positioning and layout-managed cases. Let me know if anything else should be tested.

twin._set_position(self.get_position(original=True), which="original")
twin._set_position(self.get_position(original=False), which="active")

return twin

def twinx(self, axes_class=None, **kwargs):
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24 changes: 24 additions & 0 deletions lib/matplotlib/tests/test_axes.py
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Expand Up @@ -477,6 +477,30 @@ def test_twin_inherit_autoscale_setting():
assert not ax_y_off.get_autoscaley_on()


@pytest.mark.parametrize("twin", ("x", "y"))
def test_twin_respects_position_after_set_position(twin):
fig, ax = plt.subplots()

ax.set_position([0.2, 0.2, 0.5, 0.5])
ax2 = getattr(ax, f"twin{twin}")()

assert_allclose(ax.get_position(original=True).bounds,
ax2.get_position(original=True).bounds)

assert_allclose(ax.get_position(original=False).bounds,
ax2.get_position(original=False).bounds)


@pytest.mark.parametrize("twin", ("x", "y"))
def test_twin_keeps_layout_participation_for_layout_managed_axes(twin):
fig, ax = plt.subplots()

ax2 = getattr(ax, f"twin{twin}")()

assert ax.get_in_layout()
assert ax2.get_in_layout()


def test_inverted_cla():
# GitHub PR #5450. Setting autoscale should reset
# axes to be non-inverted.
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