BUG: fix boxplot column misorder

doc/source/whatsnew/v3.0.0.rst

@@ -1354,6 +1354,7 @@ Other
 - Bug in ``divmod`` and ``rdivmod`` with :class:`DataFrame`, :class:`Series`, and :class:`Index` with ``bool`` dtypes failing to raise, which was inconsistent with ``__floordiv__`` behavior (:issue:`46043`)
 - Bug in printing a :class:`DataFrame` with a :class:`DataFrame` stored in :attr:`DataFrame.attrs` raised a ``ValueError`` (:issue:`60455`)
 - Bug in printing a :class:`Series` with a :class:`DataFrame` stored in :attr:`Series.attrs` raised a ``ValueError`` (:issue:`60568`)
+- Bug in :class: `BoxPlot` when column order was not respected in ticklabels. (:issue: `50427`)
 - Bug when calling :py:func:`copy.copy` on a :class:`DataFrame` or :class:`Series` which would return a deep copy instead of a shallow copy (:issue:`62971`)
 - Deprecated the keyword ``check_datetimelike_compat`` in :meth:`testing.assert_frame_equal` and :meth:`testing.assert_series_equal` (:issue:`55638`)
 - Fixed bug in :meth:`Series.replace` and :meth:`DataFrame.replace` when trying to replace :class:`NA` values in a :class:`Float64Dtype` object with ``np.nan``; this now works with ``pd.set_option("mode.nan_is_na", False)`` and is irrelevant otherwise (:issue:`55127`)

pandas/plotting/_matplotlib/boxplot.py

@@ -212,9 +212,10 @@ def _make_plot(self, fig: Figure) -> None:
 
                     # When `by` is assigned, the ticklabels will become unique grouped
                     # values, instead of label which is used as subtitle in this case.
-                    # error: "Index" has no attribute "levels"; maybe "nlevels"?
-                    levels = self.data.columns.levels  # type: ignore[attr-defined]
-                    ticklabels = [pprint_thing(col) for col in levels[0]]
+                    ticklabels = [
+                        pprint_thing(col)
+                        for col in self.data.columns.get_level_values(0)
+                    ]
                 else:
                     ticklabels = [pprint_thing(label)]
 

pandas/tests/plotting/test_boxplot_method.py

@@ -9,6 +9,7 @@
 import pytest
 
 from pandas import (
+    Categorical,
     DataFrame,
     MultiIndex,
     Series,
@@ -408,6 +409,20 @@ def test_boxplot_group_no_xlabel_ylabel(self, vert, request):
             )
             assert target_label == pprint_thing(["group"])
 
+    @pytest.mark.filterwarnings("ignore:set_ticklabels:UserWarning")
+    def test_boxplot_group_ordered_ticklabel(self, vert):
+        # GH 50427
+        df = DataFrame(
+            {
+                "value": [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
+                "label": ["c", "c", "c", "c", "b", "b", "b", "b", "a", "a"],
+            }
+        )
+        df.label = Categorical(df.label, categories=["c", "b", "a"], ordered=True)
+        ax = df.boxplot(by="label")
+        xticklabels = ax.get_xticklabels()
+        assert [x.get_text() for x in xticklabels] == ["c", "b", "a"]
+
 
 class TestDataFrameGroupByPlots:
     def test_boxplot_legacy1(self, hist_df):