*/
protected Graph generateGraph() {
Graph gv = new Graph();
Map<String, Graph> subgraphs = Maps.newHashMap();
gv.attr("rankdir", "TB");
Map<P2jLogicalRelationalOperator, Node> graphMap = Maps.newHashMap();
for (Entry<String, P2jLogicalRelationalOperator> e : p2jMap.entrySet()) {
Node node = new Node();
graphMap.put(e.getValue(), node);
}