Boxplot in R showing the mean

r plot boxplot
Brani picture Brani · Mar 22, 2010 · Viewed 104.4k times · Source

Does anybody know of a way of generating a boxplot in R with a line (or another symbol) in the value corresponding to the mean?

Thank you!

Answer

James picture James · Mar 22, 2010 
abline(h=mean(x))

for a horizontal line (use v instead of h for vertical if you orient your boxplot horizontally), or

points(mean(x))

for a point. Use the parameter pch to change the symbol. You may want to colour them to improve visibility too.

Note that these are called after you have drawn the boxplot.

If you are using the formula interface, you would have to construct the vector of means. For example, taking the first example from ?boxplot:

boxplot(count ~ spray, data = InsectSprays, col = "lightgray")
means <- tapply(InsectSprays$count,InsectSprays$spray,mean)
points(means,col="red",pch=18)

If your data contains missing values, you might want to replace the last argument of the tapply function with function(x) mean(x,na.rm=T)

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