presto-main/src/main/java/io/prestosql/util/GraphvizPrinter.java/GraphvizPrinter/visitSample
class GraphvizPrinter:        @Override
        public Void visitSample(SampleNode node, Void context)
        {
            printNode(node, format("Sample[type=%s, ratio=%f]", node.getSampleType(), node.getSampleRatio()), NODE_COLORS.get(NodeType.SAMPLE));
            return node.getSource().accept(this, context);
        }