Why don't you want to use any specific library ? You can't reinvent
the wheel !!
True, but Madonna is always reinventing herself...
There is a package ellipse on CRAN which will do what you are looking
for.
Have you tried
its simple geometry really:
> theta=seq(0,2*pi,len=100)
> e=1.5
> r=3
> x=e*r*cos(theta)
> y=r*sin(theta)
> plot(x,y,asp=1)
> phi=pi/4
> xr=x*sin(phi)+y*cos(phi)
> yr=-x*cos(phi)+y*sin(phi)
> plot(xr,yr,asp=1)
or something. Wrap that up into a function and you're done. This is
off-the-cuff, I've probably messed something up. So use one prepared
earlier from a library...
Baz