[R] draw a 5cm x 3cm rectangle

[Thomas Steiner]

Hi I want to draw sth in a pdf file with a predefined defined size.
Say a 5cm x 3cm rectangle (a ruler):

pdf("rect.pdf", paper="a4")
plot(c(1,5,5,1,1),c(1,1,3,3,1),asp=1,axes="n")
dev.off()

but how do I fix that one unit is 1cm?
Thanks
thomas