rank

type: rankType

• If rank="same", all nodes are placed on the same rank.
• If rank="min", all nodes are placed on the minimum rank.
• If rank="source", all nodes are placed on the minimum rank, and the only nodes on the minimum rank belong to some subgraph with rank="source" or rank="min".

Analogous criteria hold for rank="max" and rank="sink".

(Note: the minimum rank is topmost or leftmost, and the maximum rank is bottommost or rightmost.)

For more information check this answer in Stack Overflow

• Subgraphs

Note: dot only.

Search the Graphviz codebase for "rank"