Node placement in Dot

I use Dot to create callgraphs for analyzed programs. It is a very useful and powerful program. In order to clarify entry points to the program, I needed to put main and interrupt handlers all on the same level and at the top. One can do this by setting the rank attribute of the nodes to source. The code looks like this:

  { rank=source;
      main [shape=house];
      __vector_15 [shape=diamond];
      __vector_16 [shape=diamond];
      __vector_17 [shape=diamond];
  }