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

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 and X are on the same level, and so are leaf nodes B, D, and Z.
I unsuccessfully tried putting roots in one rank and leaves in another as follows:
digraph G {
rankdir = TB;
subgraph {
A -> B
A -> C
C -> D
X -> Y
rank = same; A; X;
rank = same; B; D; Y;
} /* closing subgraph */
}
And got this outcome where everything is on the same rank.
Any suggestions about what I should be trying? I've already got roots and leaves identified.

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:

Answer 2

The ideal structure is actually rank max and rank min. No need for a subgraph or any other shenanigans. GraphViz has explicit facilities for this.
With complex graphs, rank=same will often end up near the middle of the graph. If you mean top and bottom, say top and bottom.
digraph G {
rankdir = TB;
A -> B;
A -> C -> D;
X -> Y;
{ rank=min; A; X; }
{ rank=max; B; D; Y; }
}

Answer 3

Here's a simple example inspired by #William John Holden's comment -
graph {
rankdir=LR;
a -- b -- c;
d -- e -- f;
b -- d; {rank = same; b; d;};
}