positioning horizontal boxplots in ggplot2

Question

I'm trying to make a plot with horizontal boxplots in ggplot2, which you can only do by using coord_flip(). I'm also trying to space the boxplots vertically to group certain sets together. I have read that faceting is recommended for this kind of thing, but this is incompatible with coord_flip(), as we can see here: ggplot2: boxplot with facet_grid and free scale. So I'm wondering if it's possible to use blank levels in order to create blank spaces. Here's what I've managed to do so far:

d <- diamonds
library("ggplot2")
levels(d$cut) <- list(A="Fair", B="Good", "-", C="Very Good", D="Ideal", E="Premium")
p = ggplot(d, aes(x=cut, y=depth))
p + 
    geom_boxplot(color="black", size=0.2) + 
    theme_bw() + 
    scale_x_discrete(breaks = c("A", "B", "-", "C", "D", "E"), drop=FALSE) +
    coord_flip()

ph = 2.75
pw = 4
ggsave("plot.png", height=ph, width=pw)

As you can see, if I create a blank level with "-" in it and include it in scale_x_discrete(), then somehow I get a blank row. The problem is I can only add one space. Does anyone have any ideas on how to add spaces between these horizontal box plots?

Solution

Here's one way that will let you add more blank levels:

d <- diamonds
levels(d$cut) <- list(A="Fair", B="Good", " "="space1", C="Very Good", D="Ideal", "  "="space2", E="Premium")
ggplot(d, aes(x=cut, y=depth)) +
  geom_boxplot(color="black", size=0.2) + 
  theme_bw() + 
  scale_x_discrete(breaks = c("A", "B", " ", "C", "D", "  ", "E"), drop=FALSE) +
  coord_flip()

This will leave tick marks on the spacers as well, though:

Removing all ticks is simple by adding:

+ theme(axis.ticks.y = element_line(linetype=0))

But if you want to remove the tick marks only for the spacers, at least one way (I'm sure there others) is to do it with a custom function:

f <- function(x) {
  x[!x %in% c("A", "B", "C", "D", "E")] <- NA
  x
}

ggplot(d, aes(x=cut, y=depth)) +
  geom_boxplot(color="black", size=0.2) + 
  theme_bw() + 
  scale_x_discrete(breaks=f, drop=FALSE) +
  coord_flip()