BUG: Fix a bug in plotting when using color array. #20726

doc/source/whatsnew/v0.24.0.rst

@@ -1595,6 +1595,7 @@ Plotting
 
 - Bug in :func:`DataFrame.plot.scatter` and :func:`DataFrame.plot.hexbin` caused x-axis label and ticklabels to disappear when colorbar was on in IPython inline backend (:issue:`10611`, :issue:`10678`, and :issue:`20455`)
 - Bug in plotting a Series with datetimes using :func:`matplotlib.axes.Axes.scatter` (:issue:`22039`)
+- Bug in validating color parameter caused extra color to be appended to the given color array. This happened to multiple plotting functions using matplotlib. (:issue:`20726`)
 
 Groupby/Resample/Rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^

pandas/plotting/_style.py

@@ -81,7 +81,10 @@ def _maybe_valid_colors(colors):
             # mpl will raise error any of them is invalid
             pass
 
-    if len(colors) != num_colors:
+    # Append more colors by cycling if there is not enough color.
+    # Extra colors will be ignored by matplotlib if there are more colors
+    # than needed and nothing needs to be done here.
+    if len(colors) < num_colors:
         try:
             multiple = num_colors // len(colors) - 1
         except ZeroDivisionError:

pandas/tests/plotting/test_misc.py

@@ -309,3 +309,21 @@ def test_get_standard_colors_random_seed(self):
         color1 = _get_standard_colors(1, color_type='random')
         color2 = _get_standard_colors(1, color_type='random')
         assert color1 == color2
+
+    def test_get_standard_colors_no_appending(self):
+        # GH20726
+
+        # Make sure not to add more colors so that matplotlib can cycle
+        # correctly.
+        from matplotlib import cm
+        color_before = cm.gnuplot(range(5))
+        color_after = plotting._style._get_standard_colors(
+            1, color=color_before)
+        assert len(color_after) == len(color_before)
+
+        df = DataFrame(np.random.randn(48, 4), columns=list("ABCD"))
+
+        color_list = cm.gnuplot(np.linspace(0, 1, 16))
+        p = df.A.plot.bar(figsize=(16, 7), color=color_list)
+        assert (p.patches[1].get_facecolor()
+                == p.patches[17].get_facecolor())