matplotlib · tacaswell · Sep 25, 2020 · Sep 23, 2020 · Sep 24, 2020
diff --git a/doc/users/next_whats_new/errorbar_errorevery.rst b/doc/users/next_whats_new/errorbar_errorevery.rst
@@ -0,0 +1,20 @@
+``errorbar`` *errorevery* parameter matches *markevery*
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Similar to the *markevery* parameter to `~.Axes.plot`, the *errorevery*
+parameter of `~.Axes.errorbar` now accept slices and NumPy fancy indexes (which
+must match the size of *x*).
+
+.. plot::
+
+    x = np.linspace(0, 1, 15)
+    y = x * (1-x)
+    yerr = y/6
+
+    fig, ax = plt.subplots(2, constrained_layout=True)
+    ax[0].errorbar(x, y, yerr, capsize=2)
+    ax[0].set_title('errorevery unspecified')
+
+    ax[1].errorbar(x, y, yerr, capsize=2,
+                   errorevery=[False, True, True, False, True] * 3)
+    ax[1].set_title('errorevery=[False, True, True, False, True] * 3')
diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
@@ -2,7 +2,7 @@
 import itertools
 import logging
 import math
-from numbers import Number
+from numbers import Integral, Number
 
 import numpy as np
 from numpy import ma
@@ -3147,17 +3147,6 @@ def errorbar(self, x, y, yerr=None, xerr=None,
         kwargs = {k: v for k, v in kwargs.items() if v is not None}
         kwargs.setdefault('zorder', 2)
 
-        try:
-            offset, errorevery = errorevery
-        except TypeError:
-            offset = 0
-
-        if errorevery < 1 or int(errorevery) != errorevery:
-            raise ValueError(
-                'errorevery must be positive integer or tuple of integers')
-        if int(offset) != offset:
-            raise ValueError("errorevery's starting index must be an integer")
-
         self._process_unit_info([("x", x), ("y", y)], kwargs, convert=False)
 
         # Make sure all the args are iterable; use lists not arrays to preserve
@@ -3179,6 +3168,33 @@ def errorbar(self, x, y, yerr=None, xerr=None,
             if not np.iterable(yerr):
                 yerr = [yerr] * len(y)
 
+        if isinstance(errorevery, Integral):
+            errorevery = (0, errorevery)
+        if isinstance(errorevery, tuple):
+            if (len(errorevery) == 2 and
+                    isinstance(errorevery[0], Integral) and
+                    isinstance(errorevery[1], Integral)):
+                errorevery = slice(errorevery[0], None, errorevery[1])
+            else:
+                raise ValueError(
+                    f'errorevery={errorevery!r} is a not a tuple of two '
+                    f'integers')
+
+        elif isinstance(errorevery, slice):
+            pass
+
+        elif not isinstance(errorevery, str) and np.iterable(errorevery):
+            # fancy indexing
+            try:
+                x[errorevery]
+            except (ValueError, IndexError) as err:
+                raise ValueError(
+                    f"errorevery={errorevery!r} is iterable but not a valid "
+                    f"NumPy fancy index to match 'xerr'/'yerr'") from err
+        else:
+            raise ValueError(
+                f"errorevery={errorevery!r} is not a recognized value")
+
         label = kwargs.pop("label", None)
         kwargs['label'] = '_nolegend_'
 
@@ -3219,6 +3235,7 @@ def errorbar(self, x, y, yerr=None, xerr=None,
         base_style.pop('markerfacecolor', None)
         base_style.pop('markeredgewidth', None)
         base_style.pop('markeredgecolor', None)
+        base_style.pop('markevery', None)
         base_style.pop('linestyle', None)
 
         # Make the style dict for the line collections (the bars).
@@ -3260,7 +3277,7 @@ def errorbar(self, x, y, yerr=None, xerr=None,
         xuplims = np.broadcast_to(xuplims, len(x)).astype(bool)
 
         everymask = np.zeros(len(x), bool)
-        everymask[offset::errorevery] = True
+        everymask[errorevery] = True
 
         def apply_mask(arrays, mask):
             # Return, for each array in *arrays*, the elements for which *mask*

diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
@@ -3389,8 +3389,25 @@ def test_errorbar_with_prop_cycle(fig_test, fig_ref):
     ax.set_xlim(1, 11)
 
 
+def test_errorbar_every_invalid():
+    x = np.linspace(0, 1, 15)
+    y = x * (1-x)
+    yerr = y/6
+
+    ax = plt.figure().subplots()
+
+    with pytest.raises(ValueError, match='not a tuple of two integers'):
+        ax.errorbar(x, y, yerr, errorevery=(1, 2, 3))
+    with pytest.raises(ValueError, match='not a tuple of two integers'):
+        ax.errorbar(x, y, yerr, errorevery=(1.3, 3))
+    with pytest.raises(ValueError, match='not a valid NumPy fancy index'):
+        ax.errorbar(x, y, yerr, errorevery=[False, True])
+    with pytest.raises(ValueError, match='not a recognized value'):
+        ax.errorbar(x, y, yerr, errorevery='foobar')
+
+
 @check_figures_equal()
-def test_errorbar_offsets(fig_test, fig_ref):
+def test_errorbar_every(fig_test, fig_ref):
     x = np.linspace(0, 1, 15)
     y = x * (1-x)
     yerr = y/6
@@ -3401,7 +3418,7 @@ def test_errorbar_offsets(fig_test, fig_ref):
     for color, shift in zip('rgbk', [0, 0, 2, 7]):
         y += .02
 
-        # Using feature in question
+        # Check errorevery using an explicit offset and step.
         ax_test.errorbar(x, y, yerr, errorevery=(shift, 4),
                          capsize=4, c=color)
 
@@ -3411,6 +3428,27 @@ def test_errorbar_offsets(fig_test, fig_ref):
         ax_ref.errorbar(x[shift::4], y[shift::4], yerr[shift::4],
                         capsize=4, c=color, fmt='none')
 
+    # Check that markevery is propagated to line, without affecting errorbars.
+    ax_test.errorbar(x, y + 0.1, yerr, markevery=(1, 4), capsize=4, fmt='o')
+    ax_ref.plot(x[1::4], y[1::4] + 0.1, 'o', zorder=2.1)
+    ax_ref.errorbar(x, y + 0.1, yerr, capsize=4, fmt='none')
+
+    # Check that passing a slice to markevery/errorevery works.
+    ax_test.errorbar(x, y + 0.2, yerr, errorevery=slice(2, None, 3),
+                     markevery=slice(2, None, 3),
+                     capsize=4, c='C0', fmt='o')
+    ax_ref.plot(x[2::3], y[2::3] + 0.2, 'o', c='C0', zorder=2.1)
+    ax_ref.errorbar(x[2::3], y[2::3] + 0.2, yerr[2::3],
+                    capsize=4, c='C0', fmt='none')
+
+    # Check that passing an iterable to markevery/errorevery works.
+    ax_test.errorbar(x, y + 0.2, yerr, errorevery=[False, True, False] * 5,
+                     markevery=[False, True, False] * 5,
+                     capsize=4, c='C1', fmt='o')
+    ax_ref.plot(x[1::3], y[1::3] + 0.2, 'o', c='C1', zorder=2.1)
+    ax_ref.errorbar(x[1::3], y[1::3] + 0.2, yerr[1::3],
+                    capsize=4, c='C1', fmt='none')
+
 
 @image_comparison(['hist_stacked_stepfilled', 'hist_stacked_stepfilled'])
 def test_hist_stacked_stepfilled():