Fix python 3.2

pygal/graph/bar.py

@@ -78,8 +78,10 @@ class Bar(Graph):
             self._static_value(serie_node, val, x_center, y_center)
 
     def _compute(self):
-        self._box.ymin = min(self._min, self.zero)
-        self._box.ymax = max(self._max, self.zero)
+        if self._min:
+            self._box.ymin = min(self._min, self.zero)
+        if self._max:
+            self._box.ymax = max(self._max, self.zero)
 
         x_pos = [
             x / self._len for x in range(self._len + 1)