BUG: Boxplots ignore xlabel and boxplot() doesn't support horizontal layout (#45465)

* BUG: Fix support for xlabel/ylabel in boxplots.

* TST: Add tests for checking boxplot axis labels.

* DOC: Update whatsnew 1.5.0

* CLN: Add comments pointing to PR for changes.

* CLN: Default vert to True in get().

* TST: Split new test into 3 distinct tests.

* CLN: Move pprint_thing() import near other pandas imports.

* CLN: Linting.

* CLN: Add trailing comma to tests.

Author: stefmolin

doc/source/whatsnew/v1.5.0.rst

@@ -291,6 +291,9 @@ Period
 Plotting
 ^^^^^^^^
 - Bug in :meth:`DataFrame.plot.barh` that prevented labeling the x-axis and ``xlabel`` updating the y-axis label (:issue:`45144`)
+- Bug in :meth:`DataFrame.plot.box` that prevented labeling the x-axis (:issue:`45463`)
+- Bug in :meth:`DataFrame.boxplot` that prevented passing in ``xlabel`` and ``ylabel`` (:issue:`45463`)
+- Bug in :meth:`DataFrame.boxplot` that prevented specifying ``vert=False`` (:issue:`36918`)
 -
 
 Groupby/resample/rolling

pandas/plotting/_matplotlib/boxplot.py

@@ -200,7 +200,11 @@ def _make_legend(self):
         pass
 
     def _post_plot_logic(self, ax, data):
-        pass
+        # GH 45465: make sure that the boxplot doesn't ignore xlabel/ylabel
+        if self.xlabel:
+            ax.set_xlabel(pprint_thing(self.xlabel))
+        if self.ylabel:
+            ax.set_ylabel(pprint_thing(self.ylabel))
 
     @property
     def orientation(self):
@@ -237,20 +241,31 @@ def _grouped_plot_by_column(
         columns = data._get_numeric_data().columns.difference(by)
     naxes = len(columns)
     fig, axes = create_subplots(
-        naxes=naxes, sharex=True, sharey=True, figsize=figsize, ax=ax, layout=layout
+        naxes=naxes,
+        sharex=kwargs.pop("sharex", True),
+        sharey=kwargs.pop("sharey", True),
+        figsize=figsize,
+        ax=ax,
+        layout=layout,
     )
 
     _axes = flatten_axes(axes)
 
+    # GH 45465: move the "by" label based on "vert"
+    xlabel, ylabel = kwargs.pop("xlabel", None), kwargs.pop("ylabel", None)
+    if kwargs.get("vert", True):
+        xlabel = xlabel or by
+    else:
+        ylabel = ylabel or by
+
     ax_values = []
 
     for i, col in enumerate(columns):
         ax = _axes[i]
         gp_col = grouped[col]
         keys, values = zip(*gp_col)
-        re_plotf = plotf(keys, values, ax, **kwargs)
+        re_plotf = plotf(keys, values, ax, xlabel=xlabel, ylabel=ylabel, **kwargs)
         ax.set_title(col)
-        ax.set_xlabel(pprint_thing(by))
         ax_values.append(re_plotf)
         ax.grid(grid)
 
@@ -332,18 +347,28 @@ def maybe_color_bp(bp, **kwds):
         if not kwds.get("capprops"):
             setp(bp["caps"], color=colors[3], alpha=1)
 
-    def plot_group(keys, values, ax: Axes):
+    def plot_group(keys, values, ax: Axes, **kwds):
+        # GH 45465: xlabel/ylabel need to be popped out before plotting happens
+        xlabel, ylabel = kwds.pop("xlabel", None), kwds.pop("ylabel", None)
+        if xlabel:
+            ax.set_xlabel(pprint_thing(xlabel))
+        if ylabel:
+            ax.set_ylabel(pprint_thing(ylabel))
+
         keys = [pprint_thing(x) for x in keys]
         values = [np.asarray(remove_na_arraylike(v), dtype=object) for v in values]
         bp = ax.boxplot(values, **kwds)
         if fontsize is not None:
             ax.tick_params(axis="both", labelsize=fontsize)
-        if kwds.get("vert", 1):
-            ticks = ax.get_xticks()
-            if len(ticks) != len(keys):
-                i, remainder = divmod(len(ticks), len(keys))
-                assert remainder == 0, remainder
-                keys *= i
+
+        # GH 45465: x/y are flipped when "vert" changes
+        is_vertical = kwds.get("vert", True)
+        ticks = ax.get_xticks() if is_vertical else ax.get_yticks()
+        if len(ticks) != len(keys):
+            i, remainder = divmod(len(ticks), len(keys))
+            assert remainder == 0, remainder
+            keys *= i
+        if is_vertical:
             ax.set_xticklabels(keys, rotation=rot)
         else:
             ax.set_yticklabels(keys, rotation=rot)
@@ -379,6 +404,7 @@ def plot_group(keys, values, ax: Axes):
             ax=ax,
             layout=layout,
             return_type=return_type,
+            **kwds,
         )
     else:
         if return_type is None:
@@ -401,7 +427,7 @@ def plot_group(keys, values, ax: Axes):
         else:
             data = data[columns]
 
-        result = plot_group(columns, data.values.T, ax)
+        result = plot_group(columns, data.values.T, ax, **kwds)
         ax.grid(grid)
 
     return result

pandas/tests/plotting/test_boxplot_method.py

@@ -21,6 +21,7 @@
     _check_plot_works,
 )
 
+from pandas.io.formats.printing import pprint_thing
 import pandas.plotting as plotting
 
 pytestmark = pytest.mark.slow
@@ -254,6 +255,56 @@ def test_specified_props_kwd(self, props, expected):
 
         assert result[expected][0].get_color() == "C1"
 
+    @pytest.mark.parametrize("vert", [True, False])
+    def test_plot_xlabel_ylabel(self, vert):
+        df = DataFrame(
+            {
+                "a": np.random.randn(100),
+                "b": np.random.randn(100),
+                "group": np.random.choice(["group1", "group2"], 100),
+            }
+        )
+        xlabel, ylabel = "x", "y"
+        ax = df.plot(kind="box", vert=vert, xlabel=xlabel, ylabel=ylabel)
+        assert ax.get_xlabel() == xlabel
+        assert ax.get_ylabel() == ylabel
+
+    @pytest.mark.parametrize("vert", [True, False])
+    def test_boxplot_xlabel_ylabel(self, vert):
+        df = DataFrame(
+            {
+                "a": np.random.randn(100),
+                "b": np.random.randn(100),
+                "group": np.random.choice(["group1", "group2"], 100),
+            }
+        )
+        xlabel, ylabel = "x", "y"
+        ax = df.boxplot(vert=vert, xlabel=xlabel, ylabel=ylabel)
+        assert ax.get_xlabel() == xlabel
+        assert ax.get_ylabel() == ylabel
+
+    @pytest.mark.parametrize("vert", [True, False])
+    def test_boxplot_group_xlabel_ylabel(self, vert):
+        df = DataFrame(
+            {
+                "a": np.random.randn(100),
+                "b": np.random.randn(100),
+                "group": np.random.choice(["group1", "group2"], 100),
+            }
+        )
+        xlabel, ylabel = "x", "y"
+        ax = df.boxplot(by="group", vert=vert, xlabel=xlabel, ylabel=ylabel)
+        for subplot in ax:
+            assert subplot.get_xlabel() == xlabel
+            assert subplot.get_ylabel() == ylabel
+        self.plt.close()
+
+        ax = df.boxplot(by="group", vert=vert)
+        for subplot in ax:
+            target_label = subplot.get_xlabel() if vert else subplot.get_ylabel()
+            assert target_label == pprint_thing(["group"])
+        self.plt.close()
+
 
 @td.skip_if_no_mpl
 class TestDataFrameGroupByPlots(TestPlotBase):