create a boxplot in R that labels a box with the sample size (N)

Question

Is there a way to create a boxplot in R that will display with the box (somewhere) an "N=(sample size)"? The varwidth logical adjusts the width of the box on the basis of sample size, but that doesn't allow comparisons between different plots.

FWIW, I am using the boxplot command in the following fashion, where 'f1' is a factor:

boxplot(xvar ~ f1, data=frame, xlab="input values", horizontal=TRUE)

Solution 1

Here's some ggplot2 code. It's going to display the sample size at the sample mean, making the label multifunctional!

First, a simple function for fun.data

give.n <- function(x){
   return(c(y = mean(x), label = length(x)))
}

Now, to demonstrate with the diamonds data

ggplot(diamonds, aes(cut, price)) + 
   geom_boxplot() + 
   stat_summary(fun.data = give.n, geom = "text")

You may have to play with the text size to make it look good, but now you have a label for the sample size which also gives a sense of the skew.

Solution 2

You can use the names parameter to write the n next to each factor name.

If you don't want to calculate the n yourself you could use this little trick:

# Do the boxplot but do not show it
b <- boxplot(xvar ~ f1, data=frame, plot=0)
# Now b$n holds the counts for each factor, we're going to write them in names
boxplot(xvar ~ f1, data=frame, xlab="input values", names=paste(b$names, "(n=", b$n, ")"))

Solution 3

To get the n on top of the bar, you could use text with the stat details provided by boxplot as follows

b <- boxplot(xvar ~ f1, data=frame, plot=0)
text(1:length(b$n), b$stats[5,]+1, paste("n=", b$n))

The stats field of b is a matrix, each column contains the extreme of the lower whisker, the lower hinge, the median, the upper hinge and the extreme of the upper whisker for one group/plot.

Solution 4

The gplots package provides boxplot.n, which according to the documentation produces a boxplot annotated with the number of observations.

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