digraph name { ......}
dot -Tpng xxx.dot > xxx.png
digraph flow { "a" -> "b" ; "a" -> "c" ; "d" -> "e" ; "d" -> "f" ;}
graph name { ......}
neato -Tpng xxx.dot > xxx.png
graph flow { "a" -- "b" ; "a" -- "c" ; "d" -- "a" ; "d" -- "b" ;}