FIX: legend handler warning too liberal

lib/matplotlib/legend.py

@@ -1140,7 +1140,7 @@ def _get_legend_handles(axs, legend_handler_map=None):
         label = handle.get_label()
         if label != '_nolegend_' and has_handler(handler_map, handle):
             yield handle
-        elif (label not in ['_nolegend_', ''] and
+        elif (label and not label.startswith('_') and
                 not has_handler(handler_map, handle)):
             _api.warn_external(
                              "Legend does not support handles for {0} "

lib/matplotlib/tests/test_legend.py

@@ -1,6 +1,7 @@
 import collections
 import platform
 from unittest import mock
+import warnings
 
 import numpy as np
 import pytest
@@ -515,6 +516,13 @@ def test_text_nohandler_warning():
         ax.legend()
     assert len(record) == 1
 
+    # this should _not_ warn:
+    f, ax = plt.subplots()
+    ax.pcolormesh(np.random.uniform(0, 1, (10, 10)))
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        ax.get_legend_handles_labels()
+
 
 def test_empty_bar_chart_with_legend():
     """Test legend when bar chart is empty with a label."""