ellipse <- function(center=c(0,0), radius=c(1,1), angle= NULL, orient= diag(2), add=TRUE, npoints=100, ...){
# angle in radians
if (!is.null(angle)) orient=matrix(c(cos(angle), sin(angle), -1* sin(angle), cos(angle)), 2, 2)
ang=seq(from=0, to=2*pi, by =(2*pi/npoints))
z=matrix(0,(npoints+1), 2)
for (j in 1:(npoints+1)){
z[j,1]=cos(ang[j])
z[j,2]=sin(ang[j])}
z=z %*% diag(radius) %*% orient
z[,1]=z[,1]+center[1]
z[,2]=z[,2]+center[2]
if (add)
points(z[,1], z[,2], type="l", ...)
else
plot(z[,1], z[,2], type="l", ...)
}
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