code change based on review

Author: charlesdong1991

doc/source/whatsnew/v0.25.0.rst

@@ -18,7 +18,7 @@ including other versions of pandas.
 
 Other Enhancements
 ^^^^^^^^^^^^^^^^^^
-:func:`DataFrame.plot` allows to expose symlog scaling for axis.
+:func:`DataFrame.plot` keywords ``logy``, ``logx`` and ``loglog`` can now accept the value ``sym`` for symlog scaling. (:issue:`24867`)
 
 
 .. _whatsnew_0250.api_breaking:

pandas/plotting/_core.py

@@ -308,7 +308,7 @@ def _setup_subplots(self):
 
         axes = _flatten(axes)
 
-        valid_log = [False, True, 'sym', None]
+        valid_log = (False, True, 'sym', None)
 
         for i in (self.logx, self.logy, self.loglog):
             if i not in valid_log: