Backport PR matplotlib#30108: Fix constrained layout applying pad multiple times

Author: tacaswell

doc/api/next_api_changes/behavior/30108-REC.rst

@@ -0,0 +1,6 @@
+Complex layouts and constrained layout
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Constrained layout now produces smaller spacing between subplots in some
+circumstances.  This should only affect complex layouts where rows or columns
+contain different numbers of subplots, for example a layout created with
+``plt.subplot_mosaic('AC;BC', layout='constrained')``.

lib/matplotlib/_constrained_layout.py

@@ -539,14 +539,10 @@ def match_submerged_margins(layoutgrids, fig):
 
         # interior columns:
         if len(ss1.colspan) > 1:
-            maxsubl = np.max(
-                lg1.margin_vals['left'][ss1.colspan[1:]] +
-                lg1.margin_vals['leftcb'][ss1.colspan[1:]]
-            )
-            maxsubr = np.max(
-                lg1.margin_vals['right'][ss1.colspan[:-1]] +
-                lg1.margin_vals['rightcb'][ss1.colspan[:-1]]
-            )
+            leftcb = lg1.margin_vals['leftcb'][ss1.colspan[1:]]
+            rightcb = lg1.margin_vals['rightcb'][ss1.colspan[:-1]]
+            maxsubl = np.max(lg1.margin_vals['left'][ss1.colspan[1:]] + leftcb)
+            maxsubr = np.max(lg1.margin_vals['right'][ss1.colspan[:-1]] + rightcb)
             for ax2 in axs:
                 ss2 = ax2.get_subplotspec()
                 lg2 = layoutgrids[ss2.get_gridspec()]
@@ -561,22 +557,17 @@ def match_submerged_margins(layoutgrids, fig):
                         lg2.margin_vals['rightcb'][ss2.colspan[:-1]])
                     if maxsubr2 > maxsubr:
                         maxsubr = maxsubr2
-            for i in ss1.colspan[1:]:
-                lg1.edit_margin_min('left', maxsubl, cell=i)
-            for i in ss1.colspan[:-1]:
-                lg1.edit_margin_min('right', maxsubr, cell=i)
+            for i, cb in zip(ss1.colspan[1:], leftcb):
+                lg1.edit_margin_min('left', maxsubl - cb, cell=i)
+            for i, cb in zip(ss1.colspan[:-1], rightcb):
+                lg1.edit_margin_min('right', maxsubr - cb, cell=i)
 
         # interior rows:
         if len(ss1.rowspan) > 1:
-            maxsubt = np.max(
-                lg1.margin_vals['top'][ss1.rowspan[1:]] +
-                lg1.margin_vals['topcb'][ss1.rowspan[1:]]
-            )
-            maxsubb = np.max(
-                lg1.margin_vals['bottom'][ss1.rowspan[:-1]] +
-                lg1.margin_vals['bottomcb'][ss1.rowspan[:-1]]
-            )
-
+            topcb = lg1.margin_vals['topcb'][ss1.rowspan[1:]]
+            bottomcb = lg1.margin_vals['bottomcb'][ss1.rowspan[:-1]]
+            maxsubt = np.max(lg1.margin_vals['top'][ss1.rowspan[1:]] + topcb)
+            maxsubb = np.max(lg1.margin_vals['bottom'][ss1.rowspan[:-1]] + bottomcb)
             for ax2 in axs:
                 ss2 = ax2.get_subplotspec()
                 lg2 = layoutgrids[ss2.get_gridspec()]
@@ -590,10 +581,10 @@ def match_submerged_margins(layoutgrids, fig):
                             lg2.margin_vals['bottom'][ss2.rowspan[:-1]] +
                             lg2.margin_vals['bottomcb'][ss2.rowspan[:-1]]
                         ), maxsubb])
-            for i in ss1.rowspan[1:]:
-                lg1.edit_margin_min('top', maxsubt, cell=i)
-            for i in ss1.rowspan[:-1]:
-                lg1.edit_margin_min('bottom', maxsubb, cell=i)
+            for i, cb in zip(ss1.rowspan[1:], topcb):
+                lg1.edit_margin_min('top', maxsubt - cb, cell=i)
+            for i, cb in zip(ss1.rowspan[:-1], bottomcb):
+                lg1.edit_margin_min('bottom', maxsubb - cb, cell=i)
 
     return axs
 

lib/matplotlib/tests/baseline_images/test_constrainedlayout/constrained_layout12.png

@@ -809,3 +809,47 @@ def test_submerged_subfig():
     for ax in axs[1:]:
         assert np.allclose(ax.get_position().bounds[-1],
                            axs[0].get_position().bounds[-1], atol=1e-6)
+
+
+def test_submerged_height_gap():
+    """Test that the gap between rows does not depend on the number of columns."""
+
+    mosaic1 = "AC;BC"
+    mosaic2 = "ACDE;BCDE"
+
+    fig1, ax_dict1 = plt.subplot_mosaic(mosaic1, layout='constrained')
+    fig2, ax_dict2 = plt.subplot_mosaic(mosaic2, layout='constrained')
+    for fig in fig1, fig2:
+        fig.get_layout_engine().set(h_pad=0.2)
+        fig.draw_without_rendering()
+
+    for label in 'A', 'B':
+        np.testing.assert_allclose(ax_dict1[label].get_position().bounds[-1],
+                                   ax_dict2[label].get_position().bounds[-1])
+
+
+def test_submerged_width_gap():
+    """Test that the gap between columns does not depend on the number of rows."""
+
+    mosaic1 = "AB;CC"
+    mosaic2 = "AB;CC;DD"
+
+    fig1, ax_dict1 = plt.subplot_mosaic(mosaic1, layout='constrained')
+    fig2, ax_dict2 = plt.subplot_mosaic(mosaic2, layout='constrained')
+    for fig in fig1, fig2:
+        fig.get_layout_engine().set(w_pad=0.2)
+        fig.draw_without_rendering()
+
+    for label in 'A', 'B':
+        np.testing.assert_allclose(ax_dict1[label].get_position().bounds[-2],
+                                   ax_dict2[label].get_position().bounds[-2])
+
+
+@image_comparison(['test_submerged_with_colorbar.png'], style='mpl20')
+def test_submerged_with_colorbar():
+    mosaic = "AABBCC;DDDEEE"
+
+    fig, ax_dict = plt.subplot_mosaic(mosaic, layout='constrained')
+
+    cf = ax_dict['A'].contourf([[0, 1], [2, 3]])
+    fig.colorbar(cf)