[R] Adding a symbol/value/overlay to a boxplot in R

[Marc Schwartz]

On Nov 5, 2009, at 7:12 AM, Marc Giombetti wrote:

> Hello everybody,
>
> I am new to R and I am having the following problem.
> I have the following boxplot:
>
> boxplot(s$Estimate,u$Estimate,names=c("Security", "Usability"))
> title(main="Estimated development time",ylab="Estimate (weeks)")
>
> http://i35.tinypic.com/2rhw9rm.jpg
>
> but I want to add a label or symbol for a specific value to every
> boxplot. I uploaded a screenshot
> of a boxplot I generated using Excel which I try to replicate using R.
> In this example, I want to compare my dataset"usability" to a single
> value 84 (which is not part of u$Estimate). Is there a way to add a
> special symbol/value to every boxplot? Some kind of overlay layer?
>
> http://i35.tinypic.com/24n0020.jpg
>
> I appreciate your help!
>
> Thanks a lot,
> Marc


As per ?boxplot for the 'at' argument:

numeric vector giving the locations where the boxplots should be  
drawn, particularly when add = TRUE; defaults to 1:n where n is the  
number of boxes.


Thus, each box is drawn at x axis values of 1:2 in the case of your  
plots above.

You can use either text() or points() to add characters or symbols to  
the plot:

set.seed(1)
x <- rnorm(100)
grp <- rep(1:2, 50)
boxplot(x ~ grp)

text(1:2, c(-1.5, 1.75), labels = 'x')

points(1:2, c(1, -1), pch = 19)


See ?text and ?points for more information.

HTH,

Marc Schwartz