Fix missing tick labels on twinned axes.

Background:

Multi-row and/or multi-column subplots can utilize shared axes.

An external share happens at axis creation when a sharex or sharey
parameter is specified.

An internal share, or twinning, occurs when an overlayed axis is created
by the Axes.twinx() or Axes.twiny() calls.

The two types of sharing can be distinguished after the fact in the
following manner. If two axes sharing an axis also have the same
position, they are not in an external axis share, they are twinned.

For externally shared axes Pandas automatically removes tick labels for
all but the last row and/or first column in
./pandas/plotting/_matplotlib/tools.py's function _handle_shared_axes().

The problem:

_handle_shared_axes() should be interested in externally shared axes,
whether or not they are also twinned. It should, but doesn't, ignore
axes which are only twinned. Which means that twinned-only axes wrongly
lose their tick labels.

The cure:

This commit introduces _has_externally_shared_axis() which identifies
externally shared axes and uses it to expand upon the existing use of
len(Axes.get_shared_{x,y}_axes().get_siblings(a{x,y})) in
_handle_shared_axes() which miss these cases.

Author: ebardie

pandas/plotting/_matplotlib/tools.py

@@ -288,6 +288,52 @@ def _remove_labels_from_axis(axis):
     axis.get_label().set_visible(False)
 
 
+def _has_externally_shared_axis(ax1, compare_axis):
+    """
+    Return whether an axis is externally shared.
+
+    Parameters
+    ----------
+    ax1 : matplotlib.axes
+        Axis to query.
+    compare_axis : str
+        `"x"` or `"y"` according to whether the X-axis or Y-axis is being
+        compared.
+
+    Notes
+    -----
+    If two axes with different positions are sharing an axis, they can be
+    referred to as *externally* sharing the common axis.
+
+    If two axes sharing an axis also have the same position, they can be
+    referred to as *internally* sharing the common axis (a.k.a twinning).
+
+    _handle_shared_axes() is only interested in axes externally sharing an
+    axis, regardless of whether either of the axes is also internally sharing
+    with a third axis.
+    """
+    if compare_axis == "x":
+        axes = ax1.get_shared_x_axes()
+    elif compare_axis == "y":
+        axes = ax1.get_shared_y_axes()
+    else:
+        raise ValueError(
+            "_has_externally_shared_axis() needs 'x' or 'y' as a second parameter"
+        )
+
+    axes = axes.get_siblings(ax1)
+
+    # Retain ax1 and any of its siblings which aren't in the same position as it
+    ax1_points = ax1.get_position().get_points()
+    axes = [
+        ax2
+        for ax2 in axes
+        if ax2 is ax1 or not np.array_equal(ax1_points, ax2.get_position().get_points())
+    ]
+
+    return len(axes) > 1
+
+
 def _handle_shared_axes(axarr, nplots, naxes, nrows, ncols, sharex, sharey):
     if nplots > 1:
         if compat._mpl_ge_3_2_0():
@@ -311,7 +357,7 @@ def _handle_shared_axes(axarr, nplots, naxes, nrows, ncols, sharex, sharey):
                     # the last in the column, because below is no subplot/gap.
                     if not layout[row_num(ax) + 1, col_num(ax)]:
                         continue
-                    if sharex or len(ax.get_shared_x_axes().get_siblings(ax)) > 1:
+                    if sharex or _has_externally_shared_axis(ax, "x"):
                         _remove_labels_from_axis(ax.xaxis)
 
             except IndexError:
@@ -320,7 +366,7 @@ def _handle_shared_axes(axarr, nplots, naxes, nrows, ncols, sharex, sharey):
                 for ax in axarr:
                     if ax.is_last_row():
                         continue
-                    if sharex or len(ax.get_shared_x_axes().get_siblings(ax)) > 1:
+                    if sharex or _has_externally_shared_axis(ax, "x"):
                         _remove_labels_from_axis(ax.xaxis)
 
         if ncols > 1:
@@ -330,7 +376,7 @@ def _handle_shared_axes(axarr, nplots, naxes, nrows, ncols, sharex, sharey):
                 # have a subplot there, we can skip the layout test
                 if ax.is_first_col():
                     continue
-                if sharey or len(ax.get_shared_y_axes().get_siblings(ax)) > 1:
+                if sharey or _has_externally_shared_axis(ax, "y"):
                     _remove_labels_from_axis(ax.yaxis)
 
 

pandas/tests/plotting/test_misc.py

@@ -425,3 +425,53 @@ def test_dictionary_color(self):
         ax = df1.plot(kind="line", color=dic_color)
         colors = [rect.get_color() for rect in ax.get_lines()[0:2]]
         assert all(color == expected[index] for index, color in enumerate(colors))
+
+    def test_has_externally_shared_axis(self):
+        func = plotting._matplotlib.tools._has_externally_shared_axis
+
+        # Test x-axis
+        fig = self.plt.figure()
+        plots = fig.subplots(2, 4)
+
+        # Create *externally* shared axes for first and third columns
+        plots[0][0] = self.plt.subplot(231, sharex=plots[1][0])
+        plots[0][2] = self.plt.subplot(233, sharex=plots[1][2])
+
+        # Create *internally* shared axes for second and third columns
+        plots[0][1].twinx()
+        plots[0][2].twinx()
+
+        # First  column is only externally shared
+        # Second column is only internally shared
+        # Third  column is both
+        # Fourth column is neither
+        assert func(plots[0][0], "x")
+        assert not func(plots[0][1], "x")
+        assert func(plots[0][2], "x")
+        assert not func(plots[0][3], "x")
+
+        # Test y-axis
+        fig = self.plt.figure()
+        plots = fig.subplots(4, 2)
+
+        # Create *externally* shared axes for first and third rows
+        plots[0][0] = self.plt.subplot(321, sharey=plots[0][1])
+        plots[2][0] = self.plt.subplot(325, sharey=plots[2][1])
+
+        # Create *internally* shared axes for second and third rows
+        plots[1][0].twiny()
+        plots[2][0].twiny()
+
+        # First  row is only externally shared
+        # Second row is only internally shared
+        # Third  row is both
+        # Fourth row is neither
+        assert func(plots[0][0], "y")
+        assert not func(plots[1][0], "y")
+        assert func(plots[2][0], "y")
+        assert not func(plots[3][0], "y")
+
+        # Check that an invalid compare_axis value triggers the expected exception
+        msg = "needs 'x' or 'y' as a second parameter"
+        with pytest.raises(ValueError, match=msg):
+            func(plots[0][0], "z")