digraph G {
  rankdir="LR";
  label = "label demo";
  labeljust="center";
  subgraph cluster0 {
    a;
    b;
    a->b;
    label = "cluster0";
    labelloc="bottom"
  }
  subgraph cluster1 {
    c;
    d;
    c->d;
    label = "cluster1";
    labelloc="top"
  }
}