[R] Add ellipse to plot

[Alberto Monteiro]

Kamila Naxerova wrote:
>
> Is there a way to plot elliptical shapes? symbols()
> only provides circles...
>
Elipse centered at (x0, y0) with axes _a_ and _b_:

theta <- seq(0, 2 * pi, length=(big enough))
x <- x0 + a * cos(theta)
y <- y0 + b * sin(theta)
plot(x, y, type = "l")

:-)

Or, if you want to rotate it alpha degrees from the x-axis:

x <- x0 + a * cos(theta) * cos(alpha) - b * sin(theta) * sin(alpha)
y <- y0 + a * cos(theta) * sin(alpha) + b * sin(theta) * cos(alpha)

Of course, the above formulas can be rewritten without
the products of trig functions, but this way it's obvious
what it's doing.

Alberto Monteiro