Fix/mpl37 compat (#52150)

* FIX: skip Axes not in subplot layouts

As of mpl 3.7 all Axes now report as being instances of Subplots, however
ax.get_subplotspec() may now return None if the Axes is not actually in
gridspec.

* MNT: avoid unneeded indirection

The MPLPlot instance already holds a reference to the Figure, use that
instead of walking up from the first Axes.

* TST: add test of trying to layout a colorbar

Author: tacaswell

pandas/plotting/_matplotlib/core.py

@@ -1112,7 +1112,9 @@ def _get_subplots(self):
         from matplotlib.axes import Subplot
 
         return [
-            ax for ax in self.axes[0].get_figure().get_axes() if isinstance(ax, Subplot)
+            ax
+            for ax in self.fig.get_axes()
+            if (isinstance(ax, Subplot) and ax.get_subplotspec() is not None)
         ]
 
     def _get_axes_layout(self) -> tuple[int, int]:

pandas/tests/plotting/test_common.py

@@ -40,3 +40,22 @@ def test__gen_two_subplots_with_ax(self):
         subplot_geometry = list(axes[0].get_subplotspec().get_geometry()[:-1])
         subplot_geometry[-1] += 1
         assert subplot_geometry == [2, 1, 2]
+
+    def test_colorbar_layout(self):
+        fig = self.plt.figure()
+
+        axes = fig.subplot_mosaic(
+            """
+            AB
+            CC
+            """
+        )
+
+        x = [1, 2, 3]
+        y = [1, 2, 3]
+
+        cs0 = axes["A"].scatter(x, y)
+        axes["B"].scatter(x, y)
+
+        fig.colorbar(cs0, ax=[axes["A"], axes["B"]], location="right")
+        DataFrame(x).plot(ax=axes["C"])