I just spent a couple of hours trying to convert some old code that uses Mathematica 7's GraphPlot to use the new Mathematica 8 Graph functions. It seemed sensible since the new graph drawing is much nicer and it has things like AdjacencyMatrix and KirchhoffMatrix built in.

The problem is that I can not figure out how to get graphs with multiple edges to work in Mma 8.

The Feynman graph that I use as my canonical example is the two-loop vacuum graph

GraphPlot[{1 -> 2, 1 -> 2, 1 -> 2}, MultiedgeStyle -> .5, 
          DirectedEdges -> True, VertexCoordinateRules -> {{-1, 0}, {1, 0}}]

two-loop vacuum sunset graph

Trying to make the similar graph in Mma 8

Graph[{DirectedEdge[1, 2], DirectedEdge[1, 2], DirectedEdge[1, 2]}, 
      VertexCoordinates -> {{-1, 0}, {1, 0}}]

yields the error message

Graph::supp: Mixed graphs and multigraphs are not supported. >>

How can I construct (and work with) a similar graph using Mathematica 8's Graph[] objects?

Edit: This problem still exists in Mathematica 9

graph wolfram-mathematica
Edit
Report