puedes probar con la opción circle:
circle=function(R,xmin,xmax,ymin,ymax){
angle=seq(0,2*pi+0.1,0.1)
x=R*cos(angle)
y=R*sin(angle)
plot(x,y,type="l",axes=FALSE,ann=FALSE,xlim=c(xmin,xmax),ylim=c(ymin,ymax),col="mediumturquoise",asp=1,frame.plot=FALSE)}
Foro de debate
crear un circulo en un grafico
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