void ellipse (xc, yc, xr, yr, rotation, th1, th2, degrees, units) CIRCLES.C Level 3
 
float xc, yc The center point of the ellipse in the specified units.
float xr, yr The major and minor radii of the ellipse in the specified units. It is assumed that the major (long) axis of the unrotated ellipse lies on the x axis.
float rotation The rotation of the ellipse measured from the x axis.
float th1, th2 The ellipse is drawn between th1 and th2. These angles are measured with respect to the major axis of the ellipse.
char degrees = 0 Angles are in radians.
= 1 Angles are in degrees.
int units = USER_UNITS if you specify the center and radii in units of a plot.
= INCH_UNITS if you specify the center and radii in inch (or cm) units.
= PIXEL_UNITS if you specify the center and radii in pixel units.
 
With ellipse(), you can draw an ellipse segment in the specified units. To draw a circle, you would set xr 
and yr to the same number. The rotation argument lets you rotate the ellipse (in radians). The th1, th2 parameters 
give the segment of the ellipse that is drawn.