qqline connects the first and third quartiles. How do I draw a line between different quantiles (ie 30% and 70%)?

Question

I have a qqnorm plot of a data set with 1000+ points. I want to draw a line between two quantiles at a time (say 30% and 70%) just as qqline does with 25% and 75%, but with

Answer 1

EDIT:

The information below is valid for R (and stats package) version 2.15.1 - apparently later versions will incorporate the built-in capability of plotting arbitrary quantiles.

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qqline seems to be hard-coded to plot the .25 and .75 quantiles. But if you don't mind creating your own function, something like this might do:

myQqplot <- function(data, probs){
  qqnorm(data)
  theQuants <- quantile(data, probs = probs)
  lm1 <- lm(theQuants~qnorm(probs))
  abline(lm1)
  invisible()
}

myQqplot(diffbp, c(.5,.99))  

There's no check to make sure that you supply only two quantiles to the probs argument, but you could add it if you want.