Add sample size to a panel figure of boxplots - ggplot2

Question

I am trying to add sample size to boxplots (preferably at the top or bottom of them) that are grouped by two levels. I used the facet_grid() function to produce a panel plot. I then tried to use the annotate() function to add the sample sizes, however this couldn't work because it repeated the values in the second panel. Is there a simple way to do this?
head(FeatherData, n=10)
Location Status FeatherD Species ID
## 1 TX Resident -27.41495 Carolina wren CARW (32)
## 2 TX Resident -29.17626 Carolina wren CARW (32)
## 3 TX Resident -31.08070 Carolina wren CARW (32)
## 4 TX Migrant -169.19579 Yellow-rumped warbler YRWA (28)
## 5 TX Migrant -170.42079 Yellow-rumped warbler YRWA (28)
## 6 TX Migrant -158.66925 Yellow-rumped warbler YRWA (28)
## 7 TX Migrant -165.55278 Yellow-rumped warbler YRWA (28)
## 8 TX Migrant -170.43374 Yellow-rumped warbler YRWA (28)
## 9 TX Migrant -170.21801 Yellow-rumped warbler YRWA (28)
## 10 TX Migrant -184.45871 Yellow-rumped warbler YRWA (28)
ggplot(FeatherData, aes(x = Location, y = FeatherD)) +
geom_boxplot(alpha = 0.7, fill='#A4A4A4') +
scale_y_continuous() +
scale_x_discrete(name = "Location") +
theme_bw() +
theme(plot.title = element_text(size = 20, family = "Times", face =
"bold"),
text = element_text(size = 20, family = "Times"),
axis.title = element_text(face="bold"),
axis.text.x=element_text(size = 15)) +
ylab(expression(Feather~delta^2~H["f"]~"‰")) +
facet_grid(. ~ Status)

Answer 1

There's multiple ways to do this sort of task. The most flexible way is to compute your statistic outside the plotting call as a separate dataframe and use it as its own layer:
library(dplyr)
library(ggplot2)
cw_summary <- ChickWeight %>%
group_by(Diet) %>%
tally()
cw_summary
# A tibble: 4 x 2
Diet n
<fctr> <int>
1 1 220
2 2 120
3 3 120
4 4 118
ggplot(ChickWeight, aes(Diet, weight)) +
geom_boxplot() +
facet_grid(~Diet) +
geom_text(data = cw_summary,
aes(Diet, Inf, label = n), vjust = 1)
The other method is to use the summary functions built in, but that can be fiddly. Here's an example:
ggplot(ChickWeight, aes(Diet, weight)) +
geom_boxplot() +
stat_summary(fun.y = median, fun.ymax = length,
geom = "text", aes(label = ..ymax..), vjust = -1) +
facet_grid(~Diet)
Here I used fun.y to position the summary at the median of the y values, and used fun.ymax to compute an internal variable called ..ymax.. with the function length (which just counts the number of observations).