[R] drawing half-circles

[adiamond@fas.harvard.edu]

Dear wonderful R community,

I've been creating color-coded concentric circles using the "points" function, 
but I just realized that what I would really like to do is draw color-coded 
concentric half-circles.

(Because I want to communicate information about the diameters-- 
half-circles are sufficient to show diameters AND they leave ample room to one 
side of the figure to label the circles' diameter measurements.)

What should I do?

Thank you for your help,

Alexis Diamond
adiamond at fas.harvard.edu