Fix python 3.2

pygal/graph/bar.py

@@ -78,7 +78,9 @@ class Bar(Graph):
             self._static_value(serie_node, val, x_center, y_center)
 
     def _compute(self):
+        if self._min:
             self._box.ymin = min(self._min, self.zero)
+        if self._max:
             self._box.ymax = max(self._max, self.zero)
 
         x_pos = [