Merge pull request matplotlib#31353 from maf2310/fix-twin-position

Fix matplotlib#21409: Make twin axes inherit parent position

Author: QuLogic

lib/matplotlib/axes/_base.py

@@ -4725,6 +4725,20 @@ 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():
+            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):

lib/matplotlib/tests/test_axes.py

@@ -477,6 +477,42 @@ 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()
+
+
+@pytest.mark.parametrize("twin", ("x", "y"))
+def test_twin_stays_aligned_after_constrained_layout(twin):
+    fig, ax = plt.subplots(constrained_layout=True)
+
+    ax.set_position([0.2, 0.2, 0.5, 0.5])
+    ax2 = getattr(ax, f"twin{twin}")()
+
+    fig.canvas.draw()
+
+    assert_allclose(ax.get_position().bounds, ax2.get_position().bounds)
+
+
 def test_inverted_cla():
     # GitHub PR #5450. Setting autoscale should reset
     # axes to be non-inverted.