BUG: ensure that errorbar does not error on masked negative errors.

errorbar checks that errors are not negative, but a bit convolutedly,
in order to avoid triggering on nan. Unfortunately, the work-around
for nan means that possible masks get discarded, and hence passing in
a masked error array that has a negative but masked value leads to
an exception. This PR solves that by simply combining the test for
negative values with the indirect isnan test (err == err), so that
if a masked array is passed, the test values are masked and ignored
in the check.

As a bonus, this also means that astropy's ``Masked`` arrays can now
be used -- those refuse to write output of tests to unmasked values
since that would discard the mask (which is indeed the underlying
problem here).

Author: mhvk

lib/matplotlib/axes/_axes.py

@@ -3756,8 +3756,7 @@ def apply_mask(arrays, mask):
                     f"'{dep_axis}err' must not contain None. "
                     "Use NaN if you want to skip a value.")
 
-            res = np.zeros(err.shape, dtype=bool)  # Default in case of nan
-            if np.any(np.less(err, -err, out=res, where=(err == err))):
+            if np.any((err < -err) & (err == err)):
                 # like err<0, but also works for timedelta and nan.
                 raise ValueError(
                     f"'{dep_axis}err' must not contain negative values")

lib/matplotlib/tests/test_axes.py

@@ -4538,6 +4538,19 @@ def test_errorbar_nan(fig_test, fig_ref):
     ax.errorbar([4], [3], [6], fmt="C0")
 
 
+@check_figures_equal()
+def test_errorbar_masked_negative(fig_test, fig_ref):
+    ax = fig_test.add_subplot()
+    xs = range(5)
+    mask = np.array([False, False, True, True, False])
+    ys = np.ma.array([1, 2, 2, 2, 3], mask=mask)
+    es = np.ma.array([4, 5, -1, -10, 6], mask=mask)
+    ax.errorbar(xs, ys, es)
+    ax = fig_ref.add_subplot()
+    ax.errorbar([0, 1], [1, 2], [4, 5])
+    ax.errorbar([4], [3], [6], fmt="C0")
+
+
 @image_comparison(['hist_stacked_stepfilled.png', 'hist_stacked_stepfilled.png'])
 def test_hist_stacked_stepfilled():
     # make some data