Because a large portion of the pairs of a and b contain zeros in one or both conditions, they are impossible to plot as-is on a log scale.  Other MA plotting functions artificially include these condition-unique points in the plot by spreading them vertically as a "smear" on the left or horizontally as a "[https://web.archive.org/web/20110825045150/http://projetos.inpa.gov.br/i3geo/pacotes/r/win/library/graphics/html/rug.html rug]" at the very top and bottom of the plot.   In an RA plot, by contrast, the uniques are included via addition a small epsilon factor (between .1 and .5) which places them in a more statistically appropriate location in the plot.