ggplot2 facet_wrap: only use x-axis labels existing in each group

Question

I have the following dataset:

subj <- c(rep(11,3),rep(12,3),rep(14,3),rep(15,3),rep(17,3),rep(18,3),rep(20,3))
group <- c(rep("u",3),rep("t",6),rep("u",6),rep("t",6))
time <- rep(1:3,7)
mean <- c(0.7352941, 0.8059701, 0.8823529, 0.9264706, 0.9852941, 0.9558824, 0.7941176, 0.8676471, 0.7910448, 0.7058824, 0.8382353, 0.7941176, 0.9411765, 0.9558824, 0.9852941, 0.7647059, 0.8088235, 0.7968750, 0.8088235, 0.8500000, 0.8412698)
df <- data.frame(subj,group,time,mean)
df$subj <- as.factor(df$subj)
df$time <- as.factor(df$time)

And now I create a barplot with ggplot2:

library(ggplot2)
qplot(x=subj, y=mean*100, fill=time, data=df, geom="bar",stat="identity",position="dodge") +
   facet_wrap(~ group)

How do I make it so that the x-axis labels that are not present in each facet are not shown? How do I get equal distances between each subj (i.e. get rid of the bigger gaps)?

Answer 1

You can use scale="free":

ggplot(df, aes(x=subj, y=mean*100, fill=time)) +
  geom_bar(stat="identity", position="dodge") +
  facet_wrap(~ group, scale="free") 

Another option with slightly different aesthetics using facet_grid. In contrast to the plots above, the panels aren't the same width here, but due to "space="free_x", the bars are the same widths.

ggplot(df, aes(x=subj, y=mean*100, fill=time)) +
  geom_bar(stat="identity", position="dodge") +
  facet_grid(~ group, scale="free", space="free_x")