close #16593: setting ecolor turns off color cycling

doc/api/next_api_changes/behaviour.rst

@@ -93,3 +93,11 @@ Previously, `.SymLogNorm` had no *base* kwarg, and defaulted to ``base=np.e``
 whereas the documentation said it was ``base=10``.  In preparation to make
 the default 10, calling `.SymLogNorm` without the new *base* kwarg emits a
 deprecation warning.
+
+
+`~.Axes.errorbar` now color cycles when only errorbar color is set
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Previously setting the *ecolor* would turn off automatic color cycling for the plot, leading to the 
+the lines and markers defaulting to whatever the first color in the color cycle was in the case of 
+multiple plot calls. 

lib/matplotlib/axes/_axes.py

@@ -3215,8 +3215,7 @@ def errorbar(self, x, y, yerr=None, xerr=None,
             # Remove alpha=0 color that _process_plot_format returns
             fmt_style_kwargs.pop('color')
 
-        if ('color' in kwargs or 'color' in fmt_style_kwargs or
-                ecolor is not None):
+        if ('color' in kwargs or 'color' in fmt_style_kwargs):
             base_style = {}
             if 'color' in kwargs:
                 base_style['color'] = kwargs.pop('color')

lib/matplotlib/tests/test_axes.py

@@ -3157,6 +3157,21 @@ def test_errorbar_colorcycle():
     assert mcolors.to_rgba(ln1.get_color()) == mcolors.to_rgba('C2')
 
 
+@check_figures_equal()
+def test_errorbar_cycle_ecolor(fig_test, fig_ref):
+    x = np.arange(0.1, 4, 0.5)
+    y = [np.exp(-x+n) for n in range(4)]
+
+    axt = fig_test.subplots()
+    axr = fig_ref.subplots()
+
+    for yi, color in zip(y, ['C0', 'C1', 'C2', 'C3']):
+        axt.errorbar(x, yi, yerr=(yi * 0.25), linestyle='-',
+                     marker='o', ecolor='black')
+        axr.errorbar(x, yi, yerr=(yi * 0.25), linestyle='-',
+                     marker='o', color=color, ecolor='black')
+
+
 def test_errorbar_shape():
     fig = plt.figure()
     ax = fig.gca()