TST: Add test pickling plot with DatetimeIndex

Author: Licht-T

pandas/tests/plotting/test_datetimelike.py

@@ -1,6 +1,7 @@
 """ Test cases for time series specific (freq conversion, etc) """
 
 from datetime import datetime, timedelta, date, time
+import pickle
 
 import pytest
 from pandas.compat import lrange, zip
@@ -1470,5 +1471,9 @@ def _check_plot_works(f, freq=None, series=None, *args, **kwargs):
 
         with ensure_clean(return_filelike=True) as path:
             plt.savefig(path)
+
+        # GH18439
+        with ensure_clean(return_filelike=True) as path:
+            pickle.dump(fig, path)
     finally:
         plt.close(fig)