This examples demonstrates how to build a boxplot with sample size written on top of each box. It is useful to indicate what sample size is hidden behind each box. Basic R implementation.
The first challenge here is to recover the position of the top part
of each box. This is done by saving the
boxplot()
result in an object (called
boundaries
here). Now, typing
boundaries$stats
gives a dataframe with all information
concerning boxes.
Then, it is possible to use the text
function to add
labels on top of each box. This function takes 3 inputs:
boundaries$stats
object.
# Dummy data
names <- c(rep("A", 20) , rep("B", 8) , rep("C", 30), rep("D", 80))
value <- c( sample(2:5, 20 , replace=T) , sample(4:10, 8 , replace=T),
sample(1:7, 30 , replace=T), sample(3:8, 80 , replace=T) )
data <- data.frame(names,value)
# Draw the boxplot. Note result is also stored in a object called boundaries
boundaries <- boxplot(data$value ~ data$names , col="#69b3a2" , ylim=c(1,11))
# Now you can type boundaries$stats to get the boundaries of the boxes
# Add sample size on top
nbGroup <- nlevels(data$names)
text(
x=c(1:nbGroup),
y=boundaries$stats[nrow(boundaries$stats),] + 0.5,
paste("n = ",table(data$names),sep="")
)