Scatter plot with axes drawn on the same scale

I'd like to produce some scatter plots where N units on the X axis are > equal to N units on the Y axis (as measured with a ruler, on screen or paper).
x <- sample(10:200,40)
y <- sample(20:100,40)
windows(width = max(x),height = max(y))
plot(x,y)
# try:
plot(x, y, asp = 1)
# or, better:
library(MASS)
eqscplot(x,y)
#or
library(lattice)
xyplot(y ~ x, aspect = "iso")