BUG: Fix missing tick labels on twinned axes

doc/source/whatsnew/v1.2.0.rst

@@ -331,6 +331,7 @@ Plotting
 
 - Bug in :meth:`DataFrame.plot` was rotating xticklabels when ``subplots=True``, even if the x-axis wasn't an irregular time series (:issue:`29460`)
 - Bug in :meth:`DataFrame.plot` where a marker letter in the ``style`` keyword sometimes causes a ``ValueError`` (:issue:`21003`)
+- Twinned axes were losing their tick labels which should only happen to all but the last row or column of 'externally' shared axes (:issue:`33819`)
 
 Groupby/resample/rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^

pandas/plotting/_matplotlib/tools.py

@@ -297,6 +297,56 @@ def _remove_labels_from_axis(axis: "Axis"):
     axis.get_label().set_visible(False)
 
 
+def _has_externally_shared_axis(ax1: "matplotlib.axes", compare_axis: "str") -> bool:
+    """
+    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.
+
+    Returns
+    -------
+    bool
+        `True` if the axis is externally shared. Otherwise `False`.
+
+    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()
+
+    for ax2 in axes:
+        if not np.array_equal(ax1_points, ax2.get_position().get_points()):
+            return True
+
+    return False
+
+
 def handle_shared_axes(
     axarr: Iterable["Axes"],
     nplots: int,
@@ -328,7 +378,7 @@ def handle_shared_axes(
                     # 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:
@@ -337,7 +387,7 @@ def handle_shared_axes(
                 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:
@@ -347,7 +397,7 @@ def handle_shared_axes(
                 # 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

@@ -433,3 +433,117 @@ 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))
+
+    @pytest.mark.slow
+    def test_has_externally_shared_axis_x_axis(self):
+        # GH33819
+        # Test _has_externally_shared_axis() works for x-axis
+        func = plotting._matplotlib.tools._has_externally_shared_axis
+
+        fig = self.plt.figure()
+        plots = fig.subplots(2, 4)
+
+        # Create *externally* shared axes for first and third columns
+        plots[0][0] = fig.add_subplot(231, sharex=plots[1][0])
+        plots[0][2] = fig.add_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")
+
+    @pytest.mark.slow
+    def test_has_externally_shared_axis_y_axis(self):
+        # GH33819
+        # Test _has_externally_shared_axis() works for y-axis
+        func = plotting._matplotlib.tools._has_externally_shared_axis
+
+        fig = self.plt.figure()
+        plots = fig.subplots(4, 2)
+
+        # Create *externally* shared axes for first and third rows
+        plots[0][0] = fig.add_subplot(321, sharey=plots[0][1])
+        plots[2][0] = fig.add_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")
+
+    @pytest.mark.slow
+    def test_has_externally_shared_axis_invalid_compare_axis(self):
+        # GH33819
+        # Test _has_externally_shared_axis() raises an exception when
+        # passed an invalid value as compare_axis parameter
+        func = plotting._matplotlib.tools._has_externally_shared_axis
+
+        fig = self.plt.figure()
+        plots = fig.subplots(4, 2)
+
+        # Create arbitrary axes
+        plots[0][0] = fig.add_subplot(321, sharey=plots[0][1])
+
+        # 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")
+
+    @pytest.mark.slow
+    def test_externally_shared_axes(self):
+        # Example from GH33819
+        # Create data
+        df = DataFrame({"a": np.random.randn(1000), "b": np.random.randn(1000)})
+
+        # Create figure
+        fig = self.plt.figure()
+        plots = fig.subplots(2, 3)
+
+        # Create *externally* shared axes
+        plots[0][0] = fig.add_subplot(231, sharex=plots[1][0])
+        # note: no plots[0][1] that's the twin only case
+        plots[0][2] = fig.add_subplot(233, sharex=plots[1][2])
+
+        # Create *internally* shared axes
+        # note: no plots[0][0] that's the external only case
+        twin_ax1 = plots[0][1].twinx()
+        twin_ax2 = plots[0][2].twinx()
+
+        # Plot data to primary axes
+        df["a"].plot(ax=plots[0][0], title="External share only").set_xlabel(
+            "this label should never be visible"
+        )
+        df["a"].plot(ax=plots[1][0])
+
+        df["a"].plot(ax=plots[0][1], title="Internal share (twin) only").set_xlabel(
+            "this label should always be visible"
+        )
+        df["a"].plot(ax=plots[1][1])
+
+        df["a"].plot(ax=plots[0][2], title="Both").set_xlabel(
+            "this label should never be visible"
+        )
+        df["a"].plot(ax=plots[1][2])
+
+        # Plot data to twinned axes
+        df["b"].plot(ax=twin_ax1, color="green")
+        df["b"].plot(ax=twin_ax2, color="yellow")
+
+        assert not plots[0][0].xaxis.get_label().get_visible()
+        assert plots[0][1].xaxis.get_label().get_visible()
+        assert not plots[0][2].xaxis.get_label().get_visible()