presto-main/src/main/java/io/prestosql/util/GraphvizPrinter.java/GraphvizPrinter/visitUnion
class GraphvizPrinter:        @Override
        public Void visitUnion(UnionNode node, Void context)
        {
            printNode(node, "Union", NODE_COLORS.get(NodeType.UNION));

            for (PlanNode planNode : node.getSources()) {
                planNode.accept(this, context);
            }

            return null;
        }