[R] polygon

[mike.campana@freesurf.ch]

Dear all
I try once again.
In order to clearly mark values wich are larger than a treshold value, I 

would like to color the surface below the line given by plot (yy~xx). To 
color is only the surface between abline (treshold) and yy if they are 
larger than the specific limit. I guess I can use the function polygon, 
but I can not find any valuable solution.
I'm grateful to you for an advice, an example or a link.
Mike  

xx <- c(1:100)
yy <- rnorm(100)
plot (yy~xx,type="l")
abline (h=0.5,col="red")
#?? how can I use polygon()
#in order to color surface below yy value and > abline??
polygon(xx,yy,col="gray")

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