[R] plot points using circles filled half in red and half in blue.

[alan]

I want to plot many points and want to use circles. The filling color
depends on variable a. if a=1, then not fill
if a=2 then fill with red, if a=3 then fill with blue, if a=4, fill
half with red and half with blue. Can anyone tell me how to plot the
case "a=4"? Thanks a lot