From f8536e6d0de4816af29dae9e52bab86acf3dc617 Mon Sep 17 00:00:00 2001
From: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>
Date: Wed, 1 Sep 2021 04:59:36 +0200
Subject: [PATCH] Backport PR #20956: Make warning for no-handles legend more
 explicit.

---
 lib/matplotlib/legend.py | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/lib/matplotlib/legend.py b/lib/matplotlib/legend.py
index 1d85ba1a3f27..6d956a29ee48 100644
--- a/lib/matplotlib/legend.py
+++ b/lib/matplotlib/legend.py
@@ -1224,7 +1224,10 @@ def _parse_legend_args(axs, *args, handles=None, labels=None, **kwargs):
     elif len(args) == 0:
         handles, labels = _get_legend_handles_labels(axs, handlers)
         if not handles:
-            log.warning('No handles with labels found to put in legend.')
+            log.warning(
+                "No artists with labels found to put in legend.  Note that "
+                "artists whose label start with an underscore are ignored "
+                "when legend() is called with no argument.")
 
     # One argument. User defined labels - automatic handle detection.
     elif len(args) == 1: