自己的作品, generated by dot from Graphviz with the following code:

digraph ff {
  ordering=out;
  node [shape=circle]
  v1;
  v2;
  v3;
  v4;
  s -> v1;
  s -> v2;
  s -> v3 [style=invis];
  s -> v4;
  v1 -> t;
  v2 -> t [style=invis];
  v3 -> t;
  v4 -> t;
  v2 -> v1 [constraint=false, label=e1];
  v2 -> v3 [constraint=false, label=e3];
  v4 -> v3 [constraint=false, label=e2];
  { rank=source s }
  { rank=sink t }
}