Question

In this case, everything is ok:

x <- 1:10 
y <- x^3
plot(x, y)
identify(x, y)

But, using qplot there are some troubles:

x <- 1:10 
y <- x^3
qplot(x, y)
identify(x, y)

Does anybody know a similar command or other way to label specific points in ggplot2 graphs?

Answer 1

Here is a method that works using only the grid and ggplot2 packages:

library(ggplot2)
library(grid)

x <- 1:10  
y <- x^3 
qplot(x, y) 

downViewport('panel-3-4')
pushViewport(dataViewport(x,y))

tmp <- grid.locator('in')
tmp.n <- as.numeric(tmp)
tmp2.x <- as.numeric(convertX( unit(x,'native'), 'in' ))
tmp2.y <- as.numeric(convertY( unit(y,'native'), 'in' ))

w <- which.min( (tmp2.x-tmp.n[1])^2 + (tmp2.y-tmp.n[2])^2 )
grid.text(w, tmp$x, tmp$y )

If you want a text label instead of the number your could replace w in the call to grid.text with something like letters[w] (or whatever vector of labels you want).

If you are going to be doing several of these then you could wrap this in a function with the last few lines possibly in a loop. You could also add addtional logic to warn if you don’t click near a point (like identify does) or to move the label closer or further from the point (this version places the label for the nearest datapoint at the point that you click).