fix backward compatibility for str check

Author: boeddeker

pandas/plotting/_core.py

@@ -573,7 +573,7 @@ def _get_xticks(self, convert_period=False):
                 self.data = self.data[notna(self.data.index)]
                 self.data = self.data.sort_index()
                 x = self.data.index._mpl_repr()
-            elif all(isinstance(val, str) for val in index):
+            elif all(isinstance(val, string_types) for val in index):
                 x = index._mpl_repr()
             else:
                 self._need_to_set_index = True