Testgraph

<graphviz>

digraph G {

 subgraph cluster_0 {
   style=filled;
   color=lightgrey;
   node [style=filled,color=white];
   a0 -> a1 -> a2 -> a3;
   label = "process #1";
 }

 subgraph cluster_1 {
   node [style=filled];
   b0 -> b1 -> b2 -> b3;
   label = "process #2";
   color=blue
 }
 start -> a0;
 start -> b0;
 a1 -> b3;
 b2 -> a3;
 a3 -> a0;
 a3 -> end;
 b3 -> end;

 start [shape=Mdiamond];
 end [shape=Msquare];

} </graphviz>