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

mhvk · mhvk · commit c22cac0c075f · 2025-04-10T12:03:57.000-04:00
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 ensuring that the test for
negative values is stored in an array of the same type as the error
(i.e., a masked array if needed), so that the test value is 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).
diff --git a/lib/matplotlib/axes/_axes.py b/lib/matplotlib/axes/_axes.py
@@ -3756,7 +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
+            res = np.zeros_like(err, dtype=bool)  # Default in case of nan
             if np.any(np.less(err, -err, out=res, where=(err == err))):
                 # like err<0, but also works for timedelta and nan.
                 raise ValueError(
diff --git a/lib/matplotlib/tests/test_axes.py b/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
