The symbol( circle) function does not create
circles.
See following function
ijwbug <- function()
{
plot.new()
plot.window(c(-1,1),c(-1,1));
symbols(0,0,circle=1,add=T,inches=F);
curve(sqrt(1 - x^2), from = -1, to = 1, add = T, lty = 1)
curve(-sqrt(1 - x^2), from = -1, to = 1, add = T, lty = 1)
lines(c(-1,1),c(1,1));
lines(c(-1,1),c(-1,-1));
}
If I understand correctly, I think you're just seeing clipping.
Does the slight modification of your function given below behave as you
would expect ?
ijwbug <- function()
{
# do not clip to figure region, clip only to device region
par(xpd=NA)
plot.new()
plot.window(c(-1,1),c(-1,1));
symbols(0,0,circle=1,add=T,inches=F);
# I have added colour to the curves
curve(sqrt(1 - x^2), from = -1, to = 1, add = T, lty = 1, col="red")
curve(-sqrt(1 - x^2), from = -1, to = 1, add = T, lty = 1, col="red")
lines(c(-1,1),c(1,1));
lines(c(-1,1),c(-1,-1));
}
Paul