hadley/ggplot2 — comment on commit 173094a923eb2ece8c57887755eca4e45df7b67d
File: R/geom-dotplot.r line 26
By wch:

Actually, I think you should be able to just set `x=0` or `x=1`. When binning along y (and stacking along x), the dots don't have "real" x coordinates -- all the dots in a stack have the same x coordinate. The dotstack grob does the work of putting the dots next to each other. It's kind of complicated, but in a nutshell, it's because the dots have to be stacked visually, and it's not possible to do that at the geom level.