dd <- read.table(file="Cholesterol.txt",header=T)

attach(dd)

Y <- Cholesterol

res <- lm(Y ~ State + Age)

t.hat <- res$coefficients
t.hat

a1 <- t.hat[1]
a2 <- t.hat[1] + t.hat[2]
b  <- t.hat[3]

par(mai=c(0.5,0.5,0.1,0.1))
Iowa <- State == "Iowa"
Nebr <- State == "Nebraska"
plot(Age[Iowa],Y[Iowa],pch="+",xlim=c(18,78),ylim=c(100,355),xlab="",ylab="")
points(Age[Nebr],Y[Nebr],pch="x")
abline(a = a1, b = b, lwd=1, lty=3)
abline(a = a2, b = b, lwd=1, lty=2)