Tweak position of categorical data

[tongcx]

For the following example

x <- c(1,3,2,5)
y <- c("a","c","d","c")
qplot(x, y)

How do I move all points a bit downward? I can use position_jitter, but that's random. I think the best solution is to program a customized version of sth like position_jitter. However after reading source code
https://github.com/hadley/ggplot2/blob/master/R/position-jitter.r
https://github.com/hadley/ggplot2/blob/master/R/position-.r
I am still not clear how to achieve this.

[hadley]

Maybe call it position nudge?