Clarifying an error message

AllenDowney · web-flow · commit cc5c1bc14df8 · 2018-07-25T10:18:59.000-04:00
diff --git a/lib/matplotlib/axes/_base.py b/lib/matplotlib/axes/_base.py
@@ -350,7 +350,7 @@ def _plot_args(self, tup, kwargs):
         # to one element array of None which causes problems
         # downstream.
         if any(v is None for v in tup):
-            raise ValueError("x and y must not be None")
+            raise ValueError("x, y, and format string must not be None")
 
         kw = {}
         for k, v in zip(('linestyle', 'marker', 'color'),
