fix label color when the number of series is more than the number of colors available in the style

pygal/graph/graph.py

@@ -391,8 +391,7 @@ class Graph(PublicApi):
                     ) / 2,
                     width=self.legend_box_size,
                     height=self.legend_box_size,
-                    class_="color-%d reactive" % (
+                    class_="color-%d reactive" % serie_number
-                        serie_number % len(self.style.colors))
                 )
 
                 if isinstance(title, dict):