qqline in ggplot2?

[jrandall]

I am making a QQ plot using the following code:

>ggplot(data, na.rm=T) + geom_point(aes(sample=residual), stat="qq",distribution=qnorm)

How can I add a line to this plot that passes through the first and
third quartiles (such as can be done with qqline() on a qqnorm()
plot)?

[Mike Lawrence]

Compute the slope & intercept beforehand:

slope = (quantile(data$residual,p=.75)-quantile(data$residual,.25))/(qnorm(.75)-qnorm(.25))
intercept = quantile(data$residual,.25) - slope*qnorm(.25)
qq_line = data.frame(intercept=intercept,slope=slope)

Then:

ggplot(data, na.rm=T) +
geom_point(aes(sample=residual), stat="qq",distribution=qnorm) +
geom_abline(
data = qq_line
,aes(
intercept = intercept
,slope=slope
)
)

--
Mike Lawrence
Graduate Student
Department of Psychology
Dalhousie University

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~ Certainty is folly... I think. ~