[R] Draw a circle with a given radius in an existing map

[Xiaomei Ma]

I have drawn a map in which the X and Y axes are latitude and 
longitude. Now I need to draw one circle on the map - the center is a 
point with specific latitude and longitude, but the challenge is that 
the radius is in miles. Is there a way to do this? I'd very much 
appreciate your response.

XM