How can I remove empty factors from ggplot2 facets?

Question

I'm trying to modify an example of a simple forest plot by introducing facets according to a factor variable.

Assuming data of this structure:

test <- structure(list(characteristic = structure(c(1L, 2L, 3L, 1L, 2L ), .Label = c("Factor1", "Factor2", "Factor3"), class = "factor"),      es = c(1.2, 1.4, 1.6, 1.3, 1.5), ci_low = c(1.1, 1.3, 1.5,      1.2, 1.4), ci_upp = c(1.3, 1.5, 1.7, 1.4, 1.6), label = structure(c(1L,      3L, 5L, 2L, 4L), .Label = c("1.2 (1.1, 1.3)", "1.3 (1.2, 1.4)",      "1.4 (1.3, 1.5)", "1.5 (1.4, 1.6)", "1.6 (1.5, 1.7)"), class = "factor"),      set = structure(c(1L, 1L, 1L, 2L, 2L), .Label = c("H", "S"     ), class = "factor")), .Names = c("characteristic", "es",  "ci_low", "ci_upp", "label", "set"), class = "data.frame", row.names = c(NA,  -5L)) 

And running the code:

p <- ggplot(test, aes(x=characteristic, y=es, ymin=ci_low, ymax=ci_upp)) + geom_pointrange() +   coord_flip() + geom_hline(aes(x=0), lty=2) +    facet_wrap(~ set, ncol = 1) +   theme_bw() +    opts(strip.text.x = theme_text()) 

Produces output like that:

All good so far. However, I'd like to get rid of empty Factor3 level from my lower panel and cannot find a way to do that. Is there any way to do that?

Thanks for help.

Answer 1

EDIT Updated to ggplot2 0.9.3

Here's another solution. It uses facet_grid and space = "free"; also it uses geom_point() and geom_errorbarh(), and thus there is no need for coord.flip(). Also, the x-axis tick mark labels appear on the lower panel only. In the code below, the theme command is not essential - it is used to rotate the strip text to appear horizontally. Using the test dataframe from above, the following code should produce what you want:

library(ggplot2)  p <- ggplot(test, aes(y = characteristic, x = es, xmin = ci_low, xmax = ci_upp)) +    geom_point() +    geom_errorbarh(height = 0) +    facet_grid(set ~ ., scales = "free", space = "free") +    theme_bw() +    theme(strip.text.y = element_text(angle = 0))  p 

The solution is based on the example on page 124 in Wickham's ggplot2 book.