subgraph name 属性必须要以cluster开头。

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digraph {
  rankdir="LR"
  // the normal ranking algorithm doesn't know what to
  // do with clusters with rank="same".
  // If you opt in to the 'new' ranking algorithm, it
  // works as expected.
  newrank="true"

  A -> B
  A -> C
  B -> C
  C -> D

  subgraph cluster_subs {
    label="Bs and Cs"
    rank="same"
    B
    C
  }
}