How do I place nodes on the same level in DOT?

Question

I want to render several trees simultaneously and place all root nodes and all leaf nodes on the same level.

Here\'s an example of what I\'m trying to do. Root nodes A a

Answer 1

Putting the rank = same; ... statements in braces, e.g.:

digraph G { 
  rankdir = TB;
  subgraph {
    A -> B
    A -> C
    C -> D
    X -> Y
    // note that rank is used in the subgraph
    {rank = same; A; X;}
    {rank = same; B; D; Y;}
  } /* closing subgraph */
}

... gives the desired result: