[R] use different symbols for frequency in a plot

[Kerry Bush]

suppose I have the following data

x<-c(rep(.1,5),rep(.2,6),rep(.4,10),rep(.5,20))
y<-c(rep(.5,3),rep(.6,8),rep(1.2,8),rep(2.5,18),rep(3,4))

If I plot(x,y) in R, I will only get seven distinct
points. What I want to do is to use different symbols
to show the frequency at each point.

e.g. if the frequncey is between 1 and 5, then I plot
the point as a circle; if the frequency is between 6
and 10, then I plot the point as a square; if the
frequency is above 10, then I plot the point as a
triangle.

I am not sure how to do this in R. Can anybody help me?