fixed linter errors and ambigious variable names

tudplot/altair.py

@@ -1,8 +1,6 @@
 
 import altair
-from altair import Config, Color, Shape, Column, Row, Encoding, Scale, Axis
-from random import randint
-import os
+from altair import Encoding
 import logging
 
 import matplotlib.pyplot as plt
@@ -90,7 +88,7 @@ class Chart(altair.Chart):
 
     def to_mpl(self):
         d = self.to_dict()
-        fmt = 'o' if d.get('mark', 'point') is 'point' else '-'
+        fmt = 'o' if d.get('mark', 'point') == 'point' else '-'
 
         def encode(data, encoding, **kwargs):
             logging.debug(str(kwargs))