Simplify wording of allowed errorbar() error values

Author: timhoffm

lib/matplotlib/axes/_axes.py

@@ -3181,7 +3181,7 @@ def errorbar(self, x, y, yerr=None, xerr=None,
               errors.
             - *None*: No errorbar.
 
-            Note that all error arrays should have *non-negative* values.
+            All values must be >= 0.
 
             See :doc:`/gallery/statistics/errorbar_features`
             for an example on the usage of ``xerr`` and ``yerr``.
@@ -3293,9 +3293,10 @@ def has_negative_values(array):
             except TypeError:  # if array contains 'datetime.timedelta' types
                 return np.any(array < timedelta(0))
 
-        if has_negative_values(xerr) or has_negative_values(yerr):
-            raise ValueError(
-                "'xerr' and 'yerr' must have non-negative values")
+        if has_negative_values(xerr):
+            raise ValueError("'xerr' must not contain negative values")
+        if has_negative_values(yerr):
+            raise ValueError("'yerr' must not contain negative values")
 
         if isinstance(errorevery, Integral):
             errorevery = (0, errorevery)

lib/matplotlib/tests/test_axes.py

@@ -3514,18 +3514,20 @@ def test_errorbar_every_invalid():
         ax.errorbar(x, y, yerr, errorevery='foobar')
 
 
-def test_xerr_yerr_positive():
+def test_xerr_yerr_not_negative():
     ax = plt.figure().subplots()
 
-    error_message = "'xerr' and 'yerr' must have non-negative values"
-
-    with pytest.raises(ValueError, match=error_message):
+    with pytest.raises(ValueError,
+                       match="'xerr' must not contain negative values"):
         ax.errorbar(x=[0], y=[0], xerr=[[-0.5], [1]], yerr=[[-0.5], [1]])
-    with pytest.raises(ValueError, match=error_message):
+    with pytest.raises(ValueError,
+                       match="'xerr' must not contain negative values"):
         ax.errorbar(x=[0], y=[0], xerr=[[-0.5], [1]])
-    with pytest.raises(ValueError, match=error_message):
+    with pytest.raises(ValueError,
+                       match="'yerr' must not contain negative values"):
         ax.errorbar(x=[0], y=[0], yerr=[[-0.5], [1]])
-    with pytest.raises(ValueError, match=error_message):
+    with pytest.raises(ValueError,
+                       match="'yerr' must not contain negative values"):
         x = np.arange(5)
         y = [datetime.datetime(2021, 9, i * 2 + 1) for i in x]
         ax.errorbar(x=x,