DEPR: Fixed warning for implicit registration

ci/run_tests.sh

@@ -56,3 +56,7 @@ do
               bash <(curl -s https://codecov.io/bash) -Z -c -F $TYPE -f $COVERAGE_FNAME
     fi
 done
+
+# TODO: Remove this and tests/plotting/test_converter_warns.py
+echo "Running test_converter_warns.py"
+python pandas/tests/plotting/test_converter_warns.py

doc/source/whatsnew/v0.24.1.rst

@@ -67,6 +67,11 @@ Bug Fixes
 
 - Bug in :func:`merge` when merging by index name would sometimes result in an incorrectly numbered index (:issue:`24212`)
 
+**Visualization**
+
+- Fixed the warning for implicitly registered matplotlib converters not showing. See :ref:`whatsnew_0211.converters` for more (:issue:`24963`).
+
+
 **Other**
 
 -

pandas/plotting/_core.py

@@ -39,7 +39,7 @@
 else:
     _HAS_MPL = True
     if get_option('plotting.matplotlib.register_converters'):
-        _converter.register(explicit=True)
+        _converter.register(explicit=False)
 
 
 def _raise_if_no_mpl():

pandas/tests/plotting/test_converter_warning.py

@@ -0,0 +1,30 @@
+# This test ensure that we warn for implicitly registered converters.
+# We isolate it completely, because its behavior depends on some global state
+# set at import time, which is tricky to get right.
+# We use unittest instead of pytest, since pytest will import pandas when
+# loading our conftest.py
+# https://github.com/pandas-dev/pandas/issues/24963
+# https://github.com/pandas-dev/pandas/pull/18307
+# TODO: Remove the special runner in ci/run_tests.sh
+import sys
+import unittest
+
+
+class TestConverter(unittest.TestCase):
+    @unittest.skipIf("pandas" in sys.modules, "pandas musn't be imported.")
+    def test_converter_warning(self):
+        import pandas as pd
+        import pandas.util.testing as tm
+        try:
+            import matplotlib.pyplot as plt
+        except ImportError:
+            return unittest.skip("No matplotlib")
+
+        fig, ax = plt.subplots()
+        ser = pd.Series(range(12), index=pd.date_range('2000', periods=12))
+        with tm.assert_produces_warning(FutureWarning, check_stacklevel=False):
+            ax.plot(ser)
+
+
+if __name__ == '__main__':
+    unittest.main()