TST/BUG: _gen_two_subplots always adding subplot even with passed axis

pandas/tests/plotting/common.py

@@ -626,7 +626,8 @@ def _gen_two_subplots(f, fig, **kwargs):
     """
     Create plot on two subplots forcefully created.
     """
-    kwargs.get("ax", fig.add_subplot(211))
+    if "ax" not in kwargs:
+        fig.add_subplot(211)
     yield f(**kwargs)
 
     if f is pd.plotting.bootstrap_plot:

pandas/tests/plotting/test_common.py

@@ -3,7 +3,11 @@
 import pandas.util._test_decorators as td
 
 from pandas import DataFrame
-from pandas.tests.plotting.common import TestPlotBase, _check_plot_works
+from pandas.tests.plotting.common import (
+    TestPlotBase,
+    _check_plot_works,
+    _gen_two_subplots,
+)
 
 pytestmark = pytest.mark.slow
 
@@ -24,3 +28,15 @@ def test__check_ticks_props(self):
             self._check_ticks_props(ax, yrot=0)
         with pytest.raises(AssertionError, match=msg):
             self._check_ticks_props(ax, ylabelsize=0)
+
+    def test__gen_two_subplots_with_ax(self):
+        fig = self.plt.gcf()
+        gen = _gen_two_subplots(f=lambda **kwargs: None, fig=fig, ax="test")
+        # On the first yield, no subplot should be added since ax was passed
+        next(gen)
+        assert fig.get_axes() == []
+        # On the second, the one axis should match fig.subplot(2, 1, 2)
+        next(gen)
+        axes = fig.get_axes()
+        assert len(axes) == 1
+        assert axes[0].get_geometry() == (2, 1, 2)