presto-main/src/main/java/io/prestosql/util/GraphvizPrinter.java/NodePrinter/visitAggregation
class NodePrinter:        @Override
        public Void visitAggregation(AggregationNode node, Void context)
        {
            StringBuilder builder = new StringBuilder();
            for (Map.Entry<Symbol, Aggregation> entry : node.getAggregations().entrySet()) {
                builder.append(format("%s := %s\\n", entry.getKey(), formatAggregation(entry.getValue())));
            }
            printNode(node, format("Aggregate[%s]", node.getStep()), builder.toString(), NODE_COLORS.get(NodeType.AGGREGATE));
            return node.getSource().accept(this, context);
        }