axisartist: do not sort extremes if None

Author: leejjoon

lib/mpl_toolkits/axisartist/grid_helper_curvelinear.py

@@ -201,7 +201,10 @@ def get_tick_iterators(self, axes):
             xx0 = lon_levs
             dx = 0.01
 
-        e0, e1 = sorted(self._extremes)
+        if None in self._extremes:
+            e0, e1 = self._extremes
+        else:
+            e0, e1 = sorted(self._extremes)
         if e0 is None:
             e0 = -np.inf
         if e1 is None: