error while creating a simple graph

Dear All,

I am learning R so it's a very simple problem but I do not understand while
I am not able to generate a graph from two vectors.

when I type this code, it generates a very nice graph.

 pdf("mygraph.pdf")

attach(mtcars)
plot(wt,mpg)
abline(lm(mpg~wt))
title("Regreesion of mpg")
detach(mtcars)
dev.off()

But I am trying to create a graph from this code, it generates a pdf with
nothing.

dev.new()
pdf("test.pdf")
x <- c(1,3,6,9,12)
y <- c(1.5,2,7,8,15)
plot(x,y)

Thanks
Yogesh