Wow, that's actually pretty cool. =)

Thanks a lot, will try it out!

For the records, this is an decent solution for the case described in the OP

http://viz-js.com

# http://www.graphviz.org/content/cluster

digraph G {
node [shape=box]
  {
    rank = same
    j4;
    j3;
  }

j1 -> j4;
j2 -> j4;

j2 -> j5;
j7 -> j5;

j3 -> j6;

j4 -> j6;
j5 -> j6
}
[download]

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}