I'm looking for a way in R to plot graphics on map, where whatever drawn
 would be cleanly cut off at polygon(e.g country) borders. This is a picture
to illustrate what i have in mind:
http://img202.imageshack.us/img202/2672/sampleoz.png (notice that circles do
not extend over country border)
I was thinking of first plotting the circles, and then plotting the map,
with country polygons set as transparent, and the rest as white, but I
cannot seem to find a way of doing this.

Any hints on this would be greatly appreciated

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