[R] Drawing a circle on an existing graph.

[Allan Kamau]

I would like to draw a circle on a graph I am plotting in R and use
the current plot's coordinate system.
The most basic functionality I am looking for is the ability to plot a
circle on an already existing graph by simply providing the xy
coordinates for it's centre and the radius.

I am also looking for a way to obtain x,y values of a point (of a
circle) by specifying only the origin x,y the angle and radius.

Please point me so such a library and perhaps some simple one line code.

Allan.