graph G {
  fontname="Helvetica,Arial,sans-serif"
  node [fontname="Helvetica,Arial,sans-serif"]
  edge [fontname="Helvetica,Arial,sans-serif"]
  layout=fdp
  e
  subgraph clusterA {
    a -- b;
    subgraph clusterC {
      C -- D;
    }
  }
  subgraph clusterB {
    d -- f
  }
  d -- D
  e -- clusterB
  clusterC -- clusterB
}
