## to demonstrate four typical deviations from normal distribution
## chisquare distribution (skewed to the left), 
## lognormal distribution (skewed to the right), 
## t-distribution (heavy tails),
## uniform distribution (light tails)

z <- rnorm(1000)
y.chisq <- z^2
y.chisq <- (y.chisq-mean(y.chisq))/sd(y.chisq)

y.lnorm <- log(z+5)
y.lnorm <- (y.lnorm-mean(y.lnorm))/sd(y.lnorm)

y.t <- rt(1000,3)
y.t <- (y.t-mean(y.t))/sd(y.t)

y.unif <- runif(1000)
y.unif <- (y.unif-mean(y.unif))/sd(y.unif)

postscript("qqplot.ps",horizontal=T)
par(mfrow=c(2,4))

hist(y.lnorm,main="Right skewed distribution")
hist(y.chisq,main="Left skewed distribution")
hist(y.t,main="Heavy-tail distribution")
hist(y.unif,main="Light-tail distribution")

qqnorm(y.lnorm,xlim=c(-4,4),ylim=c(-4,4),main="")
abline(0,1)
qqnorm(y.chisq,ylim=c(-3,7),xlim = c(-3,7),main="")
abline(0,1)
qqnorm(y.t,xlim=c(-6,6),ylim=c(-6,6),main="")
abline(0,1)
qqnorm(y.unif,xlim=c(-3,3),ylim=c(-3,3),main="")
abline(0,1)

dev.off()