add check when two inputs have negative numbers

Author: Kislovskiy

lib/matplotlib/axes/_axes.py

@@ -3283,9 +3283,13 @@ def errorbar(self, x, y, yerr=None, xerr=None,
         x, y = np.atleast_1d(x, y)  # Make sure all the args are iterable.
         if len(x) != len(y):
             raise ValueError("'x' and 'y' must have the same size")
-        
+
         if xerr is not None and np.any(xerr < 0):
-            raise ValueError("'xerr' must have positive numbers")
+            if yerr is not None and np.any(yerr < 0):
+                raise ValueError(
+                    "'xerr' and 'yerr' must have positive numbers")
+            else:
+                raise ValueError("'xerr' must have positive numbers")
         if yerr is not None and np.any(yerr < 0):
             raise ValueError("'yerr' must have positive numbers")