% ellipse: fit ellipse to x,y data
% Usage: z=ellipse(x,y,sd);
% sd: standard deviation (default 1)
function z=ellipse(x,y,sd);
if ~exist('sd'), sd=1; end;
S=cov(x,y);
t=(0:2*pi/128:2*pi)';
xx=sin(t);
yy=cos(t);
z=[xx yy]*sd * chol(S);
z(:,1)=z(:,1)+mean(x);
z(:,2)=z(:,2)+mean(y);