Error in stat_summary(fun.y) when plotting outliers in a modified ggplot-boxplot

Question

I want to plot boxplots showing the 95 percentile instead of the IQR, including outliers as defined by exceeding the 95% criterion. This code is working fine, and based on several answers found here and on the web:

f1 <- function(x) {
  subset(x, x < quantile(x, probs=0.025)) # only for low outliers
}

f2 <- function(x) {
  r <- quantile(x, probs = c(0.025, 0.25, 0.5, 0.75, 0.975))
  names(r) <- c("ymin", "lower", "middle", "upper", "ymax")
  r
}
d <- data.frame(x=gl(2,50), y=rnorm(100))

library(ggplot2)

p0 <- ggplot(d, aes(x,y)) +
        stat_summary(fun.data = f2, geom="boxplot") + coord_flip()

p1 <- p0 + stat_summary(fun.y = f1, geom="point")

The structure of d is:

'data.frame':   100 obs. of  2 variables:
 $ x: Factor w/ 2 levels "1","2": 1 1 1 1 1 1 1 1 1 1 ...
 $ y: num  2.275 0.659 -0.821 -0.129 1.997 ...

Now, coming to my real data, which is structured essentially the same:

str(test)
'data.frame':   11830917 obs. of  2 variables:
 $ x: Ord.factor w/ 34 levels "SG26"<"SG22"<..: 18 18 18 18 18 18 18 18 18 18 ...
 $ y: num  84.6 84.1 93.3 84 93.2 94.3 83.3 92.5 94.5 98.8 ...

Now, if i am applying the same plot command, i get:

    p0 <- ggplot(test, aes(x,y)) + stat_summary(fun.data = f2, geom="boxplot") +  coord_flip() 
    p1 <- p0 + stat_summary(fun.y = f1, geom="point")
    p1

Warning message:
Computation failed in `stat_summary()`:
Argumente implizieren unterschiedliche Anzahl Zeilen: 1, 0 

The final line is the german version of "arguments imply differing number of rows 1 0". p0 is produced just fine.

What could be the difference between the two datasets?

Answer 1

The problem, as identified by @Heroka and @bdemarest, arose by one factor level having only one value.

My workaround is to skip those factors:

f1 <- function(x) {
  if (length(x) > 7) {
    return(subset(x, x < quantile(x, probs=0.025))) # only for low outliers
  } else {
    return(NA)
  }
} 

For unknown reasons, the problem persisted until there were at least 7 values per factor level.