presto-main/src/main/java/io/prestosql/util/GraphvizPrinter.java/NodePrinter/visitMarkDistinct
class NodePrinter:        @Override
        public Void visitMarkDistinct(MarkDistinctNode node, Void context)
        {
            printNode(node, format("MarkDistinct[%s]", node.getMarkerSymbol()), format("%s => %s", node.getDistinctSymbols(), node.getMarkerSymbol()), NODE_COLORS.get(NodeType.MARK_DISTINCT));
            return node.getSource().accept(this, context);
        }