FIX: labels at start of contours

Author: rcomer

lib/matplotlib/contour.py

@@ -370,7 +370,7 @@ def _split_path_and_get_label_rotation(self, path, idx, screen_pos, lw, spacing=
         # path always starts with a MOVETO, and we consider there's an implicit
         # MOVETO (closing the last path) at the end.
         movetos = (codes == Path.MOVETO).nonzero()[0]
-        start = movetos[movetos < idx][-1]
+        start = movetos[movetos <= idx][-1]
         try:
             stop = movetos[movetos > idx][0]
         except IndexError:

lib/matplotlib/tests/test_contour.py

@@ -1,6 +1,7 @@
 import datetime
 import platform
 import re
+from unittest import mock
 
 import contourpy  # type: ignore
 import numpy as np
@@ -233,6 +234,31 @@ def test_labels(split_collections):
     _maybe_split_collections(split_collections)
 
 
+def test_label_contour_start():
+    # Set up data and figure/axes that result in automatic labelling adding the
+    # label to the start of a contour
+
+    _, ax = plt.subplots(dpi=100)
+    lats = lons = np.linspace(-np.pi / 2, np.pi / 2, 50)
+    lons, lats = np.meshgrid(lons, lats)
+    wave = 0.75 * (np.sin(2 * lats) ** 8) * np.cos(4 * lons)
+    mean = 0.5 * np.cos(2 * lats) * ((np.sin(2 * lats)) ** 2 + 2)
+    data = wave + mean
+
+    cs = ax.contour(lons, lats, data)
+
+    with mock.patch.object(
+            cs, '_split_path_and_get_label_rotation',
+            wraps=cs._split_path_and_get_label_rotation) as mocked_splitter:
+        # Smoke test that we can add the labels
+        cs.clabel(fontsize=9)
+
+    # Verify at least one label was added to the start of a contour.  I.e. the
+    # splitting method was called with idx=0 at least once.
+    idxs = [cargs[0][1] for cargs in mocked_splitter.call_args_list]
+    assert 0 in idxs
+
+
 @pytest.mark.parametrize("split_collections", [False, True])
 @image_comparison(['contour_corner_mask_False.png', 'contour_corner_mask_True.png'],
                   remove_text=True, tol=1.88)