presto-main/src/main/java/io/prestosql/util/GraphvizPrinter.java/GraphvizPrinter/printFragmentNodes
class GraphvizPrinter:    private static void printFragmentNodes(StringBuilder output, PlanFragment fragment, PlanNodeIdGenerator idGenerator)
    {
        String clusterId = "cluster_" + fragment.getId();
        output.append("subgraph ")
                .append(clusterId)
                .append(" {")
                .append('\n');

        output.append(format("label = \"%s\"", fragment.getPartitioning()))
                .append('\n');

        PlanNode plan = fragment.getRoot();
        plan.accept(new NodePrinter(output, idGenerator), null);

        output.append("}")
                .append('\n');
    }