Straight edge between clusters in Graphviz

Answer 1

You can use this version :

digraph G { 
  subgraph cluster_X {
    A [ pos = "0,1!" ]; 
    B [ pos = "0,0!" ];
  } 
  subgraph cluster_Y {
    C [ pos = "1,1!" ]; 
    D [ pos = "1,0!" ];
  } 
  A -> B
  B -> C[label="yadda"]
  C -> D;
}

Then you use neato (not dot)

neato -Tpng -oyadda.png yadda.dot

And the result is :