[R] About "cex=": how to improve resolution?

[Luca Quaglia]

Hi,

I need to plot a graph with a fixed circle and with a
series of point of different size. Here is a
"simplified" example:

angle<-pi/180*c(0:360)
x<-seq(0,2,by=0.2)
y<-seq(0,2,by=0.2)
z<-seq(0,1,by=0.1)
par(pty="s")
plot(-2:2,-2:2,type="n")
lines(cos(angle),sin(angle))
points(x,y,cex=z)

The size of the points compared to the circle (of
radius 1) is important and bears a meaning. 

But instead of having 11 points with increasing size,
I only obtain points of the same size when
cex=0.1/0.2/0.3/0.4 or cex=0.5/0.6/0.7 or
cex=0.8/0.9/1.0.

Please, does anyone know if there is a way of
improving the resolution of "cex=" *without* changing
the size of the circle of radius 1 and keeping the
same axis?

Thanks in advance!!!

Luca