Add labels above top axis in ggplot2 graph while keeping original x axis on bottom

Question

I'm trying to add some labels to a ggplot2 boxplot to indicate the number of observations, and I'd like that annotation to appear above the top axis of the graph. I can add them inside the graph pretty easily, and I suspect there's an application of ggplot_gtable that might do this, but I don't understand how to use that (a point in the direction of a good tutorial would be much appreciated). Here's some example data with labels:

Count <- sample(100:500, 3)
MyData <- data.frame(Category = c(rep("A", Count[1]), rep("B", Count[2]),
                                    rep("C", Count[3])),
                     Value = c(rnorm(Count[1], 10),
                               rnorm(Count[2], 20),
                               rnorm(Count[3], 30)))
MyCounts <- data.frame(Category = c("A", "B", "C"),
                       Count = Count)
MyCounts$Label <- paste("n =", MyCounts$Count)

ggplot(MyData, aes(x = Category, y = Value)) +
      geom_boxplot() +
      annotate("text", x = MyCounts$Category, y = 35, 
               label = MyCounts$Label)

What I'd love is for the "n = 441" and other labels to appear above the graph rather than just inside the upper boundary. Any suggestions?

Answer 1

Rather than separately calculating the counts, you can add the counts with geom_text and the original data frame (MyData). The key is that we need to add stat="count" inside geom_text so that counts will be calculated and can be used as the text labels.

theme_set(theme_classic())

ggplot(MyData, aes(x = Category, y = Value)) +
  geom_boxplot() +
  geom_text(stat="count", aes(label=paste0("n=",..count..)), y=1.05*max(MyData$Value)) +
  expand_limits(y=1.05*max(MyData$Value))

To put the labels above the plot, add some space above the plot area for the text labels and then use the code in the answer linked by @aosmith to override clipping:

library(grid)

theme_set(theme_bw())

p = ggplot(MyData, aes(x = Category, y = Value)) +
  geom_boxplot() +
  geom_text(stat="count", aes(label=paste0("n=",..count..)), 
            y=1.06*max(MyData$Value), size=5) +
  theme(plot.margin=margin(t=20))

# Override clipping
gt <- ggplot_gtable(ggplot_build(p))
gt$layout$clip[gt$layout$name == "panel"] <- "off"
grid.draw(gt)