Rank constraints on the nodes in a sub-graph. 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 sub-graph whose rank attribute
is "source" or "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.