presto-main/src/main/java/io/prestosql/util/GraphvizPrinter.java/GraphvizPrinter/visitProject
class GraphvizPrinter:        @Override
        public Void visitProject(ProjectNode node, Void context)
        {
            StringBuilder builder = new StringBuilder();
            for (Map.Entry<Symbol, Expression> entry : node.getAssignments().entrySet()) {
                if ((entry.getValue() instanceof SymbolReference) &&
                        ((SymbolReference) entry.getValue()).getName().equals(entry.getKey().getName())) {
                    // skip identity assignments
                    continue;
                }
                builder.append(format("%s := %s\\n", entry.getKey(), entry.getValue()));
            }

            printNode(node, "Project", builder.toString(), NODE_COLORS.get(NodeType.PROJECT));
            return node.getSource().accept(this, context);
        }