qqplot in R

qqplot.R

# Test data: 1000 patients with two different age distributions
tdt <- data.table(age.old=rnorm(1000, mean=65, sd=15), age.young=rnorm(1000,mean=55,sd=10))

# Function to compare two distributions using qplot
qqplot <- function(x,y, data=wdt, n=100) {
    lab.x = x
    lab.y = y
    x <- with(data, get(x))
    x <- sapply(seq(0,1,1/n), function(q) quantile(x, q, na.rm=TRUE))
    y <- with(data, get(y))
    y <- sapply(seq(0,1,1/n), function(q) quantile(y, q, na.rm=TRUE))
    # Get axes symmetrical
    axes.minmax <- c(floor(min(c(x,y))), ceiling(max(c(x,y))))
    # print(axes.minmax)
    qplot(x,y, asp=1, xlab=lab.x, ylab=lab.y) +
        geom_abline(intercept=0,slope=1) +
        coord_cartesian(x=axes.minmax, y=axes.minmax)
}

qqplot('age.young', 'age.old', data=tdt)

thanks for sharing; fairly obvious, but might be helpful for some to include

library(data.table)
library(ggplot2)

at the beginning.