Make warning for no-handles legend more explicit.

anntzer · anntzer · commit fc7bc44dc5aa · 2021-08-31T21:33:33.000+02:00
It may be slightly better to only emit the longer message if there were
actually handles that got ignored because of the leading underscore, but
that's actually a bit annoying to check (or requires some refactoring of
_get_legend_handles_labels).
diff --git 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:
