From cc5c1bc14df88e49584f888c9f7b25cd273b5d40 Mon Sep 17 00:00:00 2001
From: Allen Downey <downey@allendowney.com>
Date: Wed, 25 Jul 2018 10:18:59 -0400
Subject: [PATCH] Clarifying an error message

---
 lib/matplotlib/axes/_base.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/matplotlib/axes/_base.py b/lib/matplotlib/axes/_base.py
index 4b9bdf9c7245..f1e1116a5a58 100644
--- 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'),