The graph depicts the genomic distribution of alternating pyrimidine/purine (YR) stretches of 10 bp or more in prokaryotic phyla. The expected fraction is 0.001 (0.1%). It can be seen that the β-Proteobacterial group has more than the expected fractions of YR-stretches, while all other groups have, on average, less than expected.
Bohlin et al. BMC Genomics 2009 10:346 doi:10.1186/1471-2164-10-346