Make sure custom alpha param does not change 'none' colors in a list of colors (matplotlib#27845)

* fix issue 27839, make sure alpha param does not affect none colors
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Co-authored-by: Tim Hoffmann <[email protected]>

* Update lib/matplotlib/colors.py

Co-authored-by: Elliott Sales de Andrade <[email protected]>

---------

Co-authored-by: Tim Hoffmann <[email protected]>
Co-authored-by: Elliott Sales de Andrade <[email protected]>

Author: 3 people

lib/matplotlib/colors.py

@@ -506,6 +506,11 @@ def to_rgba_array(c, alpha=None):
 
     if alpha is not None:
         rgba[:, 3] = alpha
+        if isinstance(c, Sequence):
+            # ensure that an explicit alpha does not overwrite full transparency
+            # for "none"
+            none_mask = [cbook._str_equal(cc, "none") for cc in c]
+            rgba[:, 3][none_mask] = 0
     return rgba
 
 

lib/matplotlib/tests/test_colors.py

@@ -19,6 +19,7 @@
 import matplotlib.scale as mscale
 from matplotlib.rcsetup import cycler
 from matplotlib.testing.decorators import image_comparison, check_figures_equal
+from matplotlib.colors import to_rgba_array
 
 
 @pytest.mark.parametrize('N, result', [
@@ -1686,3 +1687,13 @@ def test_set_cmap_mismatched_name():
     cmap_returned = plt.get_cmap("wrong-cmap")
     assert cmap_returned == cmap
     assert cmap_returned.name == "wrong-cmap"
+
+
+def test_to_rgba_array_none_color_with_alpha_param():
+    # effective alpha for color "none" must always be 0 to achieve a vanishing color
+    # even explicit alpha must be ignored
+    c = ["blue", "none"]
+    alpha = [1, 1]
+    assert_array_equal(
+        to_rgba_array(c, alpha), [[0., 0., 1., 1.], [0., 0., 0., 0.]]
+    )