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

The y axis is meaningless. It just needs to have _some_ value for ggplot to be happy, so I had the geom set it to [0,1]. The reason it's meaningless is because it's not possible to set the y range relative to the visual height of the dot. It's kind of hard to explain... I think it's easier to understand if you just make a plot in a window, and then play around with resizing the window. (Keep in mind that the y range can't change in response to resizing the window.)