geom_boxplot() from ggplot2 : forcing an empty level to appear

Question

I can\'t find a way to ask ggplot2 to show an empty level in a boxplot without imputing my dataframe with actual missing values. Here is reproducible code :

Answer 1

You can control the breaks in a suitable scale function, in this case scale_x_discrete. Make sure you use the argument drop=FALSE:

p <- ggplot(data=dftest[dftest$time!=2,],aes(x=factor(time,levels=1:10),y=value))
p + geom_boxplot() + 
  scale_x_discrete("time", breaks=factor(1:10), drop=FALSE)

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I like to do my data manipulation in advance of sending it to ggplot. I think this makes the code more readable. This is how I would do it myself, but the results are the same. Note, however, that the ggplot scale gets much simpler, since you don't have to specify the breaks:

dfplot <- dftest[dftest$time!=2, ]
dfplot$time <- factor(dfplot$time, levels=1:10)

ggplot(data=dfplot, aes(x=time ,y=value)) +
    geom_boxplot() + 
    scale_x_discrete("time", drop=FALSE)