The plot represents stringency of nucleosome positioning (at the dyad) as a fraction of positioned nucleosomes. Stringency metric is given by a ratio of putative mononucleosome dyad instances falling in a 23 base pair window of a given site to the number of infringing nucleosome dyads in a 300 base pair window.
Valouev A, Ichikawa J, Tonthat T, Stuart J, Ranade S, Peckham H, Zeng K, Malek JA, Costa G, McKernan K, Sidow A, Fire A, Johnson SM. A high-resolution, nucleosome position map of C. elegans reveals a lack of universal sequence-dictated positioning. Genome Res. 2008 Jul;18(7):1051-63. Epub 2008 May 13.
Supported oligo ligation detection (SOLiD) sequencing technology from Applied Biosystems.