voltage graphs: fix negative values

graph_tools/voltage_graphs.py

@@ -97,7 +97,7 @@ def _init_():
       else:
         for i in range(-a):
           gadget.out(v if i == 0 else free_edges[i-1])
-          gadget.out(free_edges[i] if i < a - 1 else u)
+          gadget.out(free_edges[i] if i < abs(a) - 1 else u)
 
     gadget = gadget.finish()