[R] drawing segments through points with pch=1

[David Epstein]

Please excuse me for asking such basic questions:

Here is my code
> y=c(1.21,0.51,0.14,1.62,-0.8,0.72,-1.71,0.84,0.02,-0.12)
> ybar=mean(y)
> ll=length(y);
> ybarv=rep(ybar,ll)
> x=1:ll
> plot(x,ybarv,pch=1)
> segments(x[1],ybar,x[ll],ybar)

What I get is a collection of small circles, with a segment "on top"  
of the circles, which is almost what I want. But I don't want the  
segment to be visible inside any small circle.

Is there an easy way to arrange for the segment to "lie behind" the  
pch=1 markers, as in hidden line removal, so that the circles remain  
with nothing inside them? I tried putting the segments command first,  
but then no segment appeared at all.

In general, is there a method of laying a drawing "on top" of  
another. I tried inserting add="T" as an argument to plot, and R  
objected strongly.

Thanks for any help

David Epstein