How to obtain values (e.g. median) from a boxplot in r?

Question

I’ve plotted a boxplot for PM2.5 levels per year.

Boxplot(PM2.5~year, data=subset(dat, hour==12), las=1)

How can I extract values such as the median from the boxplots?

Answer 1

The default boxplot function returns summaries invisibly, you just have to assign it to a variable:

res <- boxplot(Sepal.Length ~ Species, data=iris)

Within res there exists an element stats:

> res$stats
     [,1] [,2] [,3]
[1,]  4.3  4.9  5.6
[2,]  4.8  5.6  6.2
[3,]  5.0  5.9  6.5
[4,]  5.2  6.3  6.9
[5,]  5.8  7.0  7.9

These are quartile summaries of the boxes. The median is the middle one, so:

> res$stats[3,]
[1] 5.0 5.9 6.5