fix wording & simplify function

Author: Kislovskiy

lib/matplotlib/axes/_axes.py

@@ -3180,7 +3180,7 @@ def errorbar(self, x, y, yerr=None, xerr=None,
               errors.
             - *None*: No errorbar.
 
-            Note that all error arrays should have *positive* values.
+            Note that all error arrays should have *non-negative* values.
 
             See :doc:`/gallery/statistics/errorbar_features`
             for an example on the usage of ``xerr`` and ``yerr``.
@@ -3284,17 +3284,21 @@ def errorbar(self, x, y, yerr=None, xerr=None,
         if len(x) != len(y):
             raise ValueError("'x' and 'y' must have the same size")
 
-        def check_if_negative(array):
+        def has_negative_values(array):
             if array is None:
                 return False
             try:
+                return np.any(array < 0)
+            except TypeError:
+                pass   # Don't fail on 'datetime.*' types
                 if np.any(array < 0):
                     return True
             except TypeError:   # Don't fail on 'datetime.*' types
                 pass
 
-        if check_if_negative(xerr) or check_if_negative(yerr):
-            raise ValueError("'xerr' and 'yerr' must have positive numbers")
+        if has_negative_values(xerr) or has_negative_values(yerr):
+            raise ValueError(
+                "'xerr' and 'yerr' must have non-negative numbers")
 
         if isinstance(errorevery, Integral):
             errorevery = (0, errorevery)

lib/matplotlib/tests/test_axes.py

@@ -3516,14 +3516,13 @@ def test_errorbar_every_invalid():
 def test_xerr_yerr_positive():
     ax = plt.figure().subplots()
 
-    with pytest.raises(ValueError,
-                       match="'xerr' and 'yerr' must have positive numbers"):
+    error_message = "'xerr' and 'yerr' must have non-negative numbers"
+
+    with pytest.raises(ValueError, match=error_message):
         ax.errorbar(x=[0], y=[0], xerr=[[-0.5], [1]], yerr=[[-0.5], [1]])
-    with pytest.raises(ValueError,
-                        match="'xerr' and 'yerr' must have positive numbers"):
+    with pytest.raises(ValueError, match=error_message):
         ax.errorbar(x=[0], y=[0], xerr=[[-0.5], [1]])
-    with pytest.raises(ValueError,
-                        match="'xerr' and 'yerr' must have positive numbers"):
+    with pytest.raises(ValueError, match=error_message):
         ax.errorbar(x=[0], y=[0], yerr=[[-0.5], [1]])