Partly revert matplotlib#27711

This PR removes the propagation of `labels` to any artist legend labels.

Other than the rest of the plotting functions `labels` is not used for legend labels
but for xtick labels. This is only poorly documented via
https://matplotlib.org/stable/api/_as_gen/matplotlib.axes.Axes.bxp.html and in an
[example](https://matplotlib.org/stable/gallery/statistics/boxplot_color.html).

Whatever our way forward regarding the use of `labels` is, we should by no means
propagate them simultaneously to xticks and legend entries. This coupling would cripple
users' configurability and limit our ability to migrate to a clear API where legend
labels and tick labels can be configured independently.

Until we have sorted out a better API, the recommended solution for the original issue
matplotlib#20512 is to grab the artists returned from `boxplot()` and either `set_label()` on
them or pass them to the legend call `ax.legend(handles, labels)`.

Author: timhoffm

lib/matplotlib/axes/_axes.py

@@ -3885,7 +3885,8 @@ def boxplot(self, x, notch=None, sym=None, vert=None, whis=None,
             boxes are drawn with Patch artists.
 
         labels : sequence, optional
-            Labels for each dataset (one per dataset).
+            Labels for each dataset (one per dataset). These are used for
+            x-tick labels; *not* for legend entries.
 
         manage_ticks : bool, default: True
             If True, the tick locations and labels will be adjusted to match
@@ -4004,9 +4005,6 @@ def boxplot(self, x, notch=None, sym=None, vert=None, whis=None,
             if 'color' in boxprops:
                 boxprops['edgecolor'] = boxprops.pop('color')
 
-        if labels:
-            boxprops['label'] = labels
-
         # if non-default sym value, put it into the flier dictionary
         # the logic for providing the default symbol ('b+') now lives
         # in bxp in the initial value of flierkw

lib/matplotlib/tests/test_legend.py

@@ -1429,31 +1429,16 @@ def test_legend_text():
     assert_allclose(leg_bboxes[1].bounds, leg_bboxes[0].bounds)
 
 
-def test_boxplot_legend():
-    # Test that boxplot legends handles are patches
-    # and labels are generated from boxplot's labels parameter.
-    fig, axs = plt.subplots()
-    A = 5*np.random.rand(100, 1)
-    B = 10*np.random.rand(100, 1) - 5
-    C = 7*np.random.rand(100, 1) - 5
-    labels = ['a', 'b', 'c']
-
-    bp0 = axs.boxplot(A, positions=[0], patch_artist=True, labels=labels[0])
-    bp1 = axs.boxplot(B, positions=[1], patch_artist=True, labels=labels[1])
-    bp2 = axs.boxplot(C, positions=[2], patch_artist=True, labels=labels[2])
-    # red, blue, green
-    colors = [(1.0, 0.0, 0.0, 1), (0.0, 0.0, 1.0, 1), (0.0, 0.5, 0.0, 1)]
-    box_list = [bp0, bp1, bp2]
-    # Set colors to the boxes
-    lbl_index = 0
-    for b_plot, color in zip(box_list, colors):
-        for patch in b_plot['boxes']:
-            patch.set_color(color)
-            lbl_index += 1
-
-    legend = axs.legend()
-    for index, handle in enumerate(legend.legend_handles):
-        assert isinstance(handle, mpl.patches.Rectangle)
-        assert handle.get_facecolor() == colors[index]
-        assert handle.get_edgecolor() == colors[index]
-        assert handle.get_label() == labels[index]
+def test_boxplot_labels():
+    # Test that boxplot(..., labels=) sets the tick labels but not legend entries
+    # This is not consistent with other plot types but is the current behavior.
+    fig, ax = plt.subplots()
+    np.random.seed(19680801)
+    data = np.random.random((10, 3))
+    bp = ax.boxplot(data, labels=['A', 'B', 'C'])
+    # Check that labels set the tick labels ...
+    assert [l.get_text() for l in ax.get_xticklabels()] == ['A', 'B', 'C']
+    # ... but not legend entries
+    handles, labels = ax.get_legend_handles_labels()
+    assert len(handles) == 0
+    assert len(labels) == 0