Backport PR matplotlib#31943: FIX: allow non-strict monotonicity in LinearSegmented.from_list values

Author: jklymak

lib/matplotlib/colors.py

@@ -1233,7 +1233,7 @@ def from_list(name, colors, N=256, gamma=1.0, *, bad=None, under=None, over=None
             except Exception as e2:
                 raise e2 from e
             vals = np.asarray(_vals)
-            if np.min(vals) < 0 or np.max(vals) > 1 or np.any(np.diff(vals) <= 0):
+            if np.min(vals) < 0 or np.max(vals) > 1 or np.any(np.diff(vals) < 0):
                 raise ValueError(
                     "the values passed in the (value, color) pairs "
                     "must increase monotonically from 0 to 1."

lib/matplotlib/tests/test_colors.py

@@ -1828,6 +1828,13 @@ def test_LinearSegmentedColormap_from_list_value_color_tuple():
     )
 
 
+def test_LinearSegmentedColormap_from_list_repeat_values():
+    # Smoke test that we can pass repeated values to make a solid band of color
+    # followed by a linear ramp.
+    value_color_tuples = [(0, "red"), (0.6, "red"), (0.6, "blue"), (1, "green")]
+    mcolors.LinearSegmentedColormap.from_list("lsc", value_color_tuples, N=11)
+
+
 @image_comparison(['test_norm_abc.png'], remove_text=True, style='_classic_test',
                   tol=0 if platform.machine() == 'x86_64' else 0.05)
 def test_norm_abc():