.ND
.pl 100i
.po 3
.na
.SH
draw circles and ellipses

.SH
Usage:
.LP
.nf
.in +0.5i
circle(x=0, y=0, radius=1, ...)ellipse(x=0, y=0, sx=1, sy=1,
rho=0, fine=0.01, center=0.1, add=T, show=T, ...)norm.ellipse(x, y,
group, homosked=T, level=0.05,   radius=sqrt(qchisq(1-level,2)), ...)
.in -0.5i
.fi

.SH
Arguments:
.IP "" 0.5i

.IP "" 1i
.ti -\w@x,y:\ @u
x,y:\ center of circle or ellipse before
orienting

.IP "" 1i
.ti -\w@radius:\ @u
radius:\ length of radius (positive)

.IP "" 1i
.ti -\w@sx,sy,rho:\ @u
sx,sy,rho:\ standardizing units for x,y (positive)and
correlation between x and y (between -1 and 1)

.IP "" 1i
.ti -\w@fine:\ @u
fine:\ fineness of arcs for drawing (between 0 and 1)

.IP "" 1i
.ti -\w@center:\ @u
center:\ if positive, fraction of radius for main axes

.IP "" 1i
.ti -\w@show:\ @u
show:\ plot ellipse or circle if TRUE

.IP "" 1i
.ti -\w@add:\ @u
add:\ add to existing plot if TRUE

.IP "" 1i
.ti -\w@orient:\ @u
orient:\ orientation of ellipse

.IP "" 1i
.ti -\w@xo,yo:\ @u
xo,yo:\ offset of origin

.IP "" 1i
.ti -\w@\&...:\ @u
\&...:\ plot parameters (many are set if add=F)


.SH
Value:
.IP "" 0.5i
list of dataframes containing ellipse or circle arc(s)

.SH
Examples:
.LP
.nf
.in +0.5i

# circle with unit radius
circle()
# return coordinates of arc for ellipse
ellipse(sx=2,sy=3,rho=.5,show=F)
# ellipse rotated 45 degrees, centered at (1,2)
ellipse(xo=1,yo=2,sx=.25,sy=.5,orient=pi/2)
# ellipse at (1,2), but then rotated 45 degrees
ellipse(x=1,y=2,sx=.25,sy=.5,orient=pi/2)
# 95% contour for bivariate normal from data (x,y)
norm.ellipse(x,y)
.in -0.5i
.fi