Plot quantiles of distribution in ggplot2 with facets

Question

I'm currently plotting a number of different distributions of first differences from a number of regression models in ggplot. To facilitate interpretation of the differences, I want to mark the 2.5% and the 97.5% percentile of each distribution. Since I will be doing quite a few plots, and because the data is grouped in two dimension (model and type), I would like to define and plot the respective percentiles in the ggplot environment. Plotting the distributions using facets gets me to exactly where I want except for the percentiles. I could of course do this more manually, but I would ideally want to find a solution where I am still able to use facet_grid, since this spared me a lot of hassle trying to fit the different plots together.

Here's an example using simulated data:

df.example <- data.frame(model = rep(c("a", "b"), length.out = 500), 
                      type = rep(c("t1", "t2", "t2", "t1"), 
                      length.outh = 250), value = rnorm(1000))

 ggplot(df.example, aes(x = value)) +
 facet_grid(type ~ model) +
 geom_density(aes(fill = model, colour = model))

I've tried to add quantiles in two ways. The first one produces an error message:

 ggplot(df.example, aes(x = value)) +
 facet_grid(. ~ model) +
 geom_density(aes(fill = model, colour = model)) +
 geom_vline(aes(x = value), xintercept = quantile(value, probs = c(.025, .975)))

Error in quantile(value, probs = c(0.025, 0.975)) : object 'value' not found

While the second one gets me the quantiles for the the complete variable and not for the sub-densities. That is, the plotted quantiles are identical for all four densities.

 ggplot(df.example, aes(x = value)) +
 facet_grid(type ~ model) +
 geom_density(aes(fill = model, colour = model)) +
 geom_vline(xintercept = quantile(df.example$value, probs = c(.025, .975)))

I consequently wonder if there is a way to plot the specific quantiles for each subgroup within the ggplot2 environment?

Greatly appreciate any input.

Answer 1

You can calculate the quantiles beforehand.

Using your example data:

library (dplyr)
d2 <- df.example %>%
  group_by(model, type) %>%
  summarize(lower = quantile(value, probs = .025),
            upper = quantile(value, probs = .975))

And then plot like this:

ggplot(df.example, aes(x = value)) +
  facet_grid(type ~ model) +
  geom_density(aes(fill = model, colour = model)) +
  geom_vline(data = d2, aes(xintercept = lower)) +
  geom_vline(data = d2, aes(xintercept = upper))

Answer 2

Use plyr (or dplyr, data.table) to precompute these values ...

set.seed(1)
# ...

df.q <- ddply(df.example, .(model, type),
              summarize, q=quantile(value, c(.025, .975)))    
p + geom_vline(aes(xintercept=q), data=df.q)

Answer 3

Good question. The more general version of the same question is: how do you call functions on the subsetted datasets when using facets? This seems like a very useful feature and so I searched around but could not find anything about it.

The answers already given are excellent. Another option is to use multiplot() as a way of doing the faceting manually.