How to enforce ggplot's position_dodge on categories with no data?

Question

I\'m trying to use position_dodge on ggplot to obtain boxplots of two different signals (ind) sharing the same categories (cat). When there is a category with data for one

Answer 1

After some workarounds, I came up with the outcome I was looking for... (kind of)

data            <- data.frame(
cat=c('A','A','A','A','B','B','A','A','A','A','B','B','B'), 
values=c(3,2,1,4,NA,NA,4,5,6,7,8,9, 0), 
ind=c('x','x','x','x','x','x','y','y','y','y','y','y','x'))

p  <- ggplot() +
      scale_colour_hue(guide='none') +
      geom_boxplot(aes(x=as.factor(cat), y=values, fill=ind),
      position=position_dodge(width=.60), 
      data=data,
      outlier.size = 1.2,
      na.rm=T) +
      geom_line(aes(x=x, y=y), 
                data=data.frame(x=c(0,3),y=rep(0,2)), 
                size = 1, 
                col='white')
print(p)

Some people recommended using faceting for the effect I wanted. Faceting doesn't give me the effect I'm looking for. The final graph I was looking for is shown below:

If you notice, the white major tick mark at y = 10 is thicker than the other tick marks. This thicker line is the geom_line with size=1 that hides unwanted boxplots.

I wish we could combine different geom objects more seamlessly. I reported this as a bug on Hadley's github, but Hadley said this is how position_dodge behaves by design. I guess I'm using ggplot2 in a non-standard way and workarounds are the way to go on these kind of issues. Anyways, I hope this helps some of the R folks to push ggplot great functionality a little further.