function stem3(n,s,color);
% function stem3(n,s,color);
%
% stems in 3d coordinates. color might be 'r', 'g', or the like. 
% n and s should be line vectors. 
%
% does not work yet with the color ... 

if nargin == 2,  % default color is blue. 
  color='b'; 
end
m = length(s);
x = n'; 
xx = zeros(3*m,1);
xx(1:3:3*m,:) = x;
xx(2:3:3*m,:) = x;
xx(3:3:3*m,:) = NaN;

y = real(s)'; 
yy = zeros(3*m,1);
yy(2:3:3*m,:) = y;
yy(3:3:3*m,:) = NaN;

z = imag(s)'; 
zz = zeros(3*m,1);
zz(2:3:3*m,:) = z;
zz(3:3:3*m,:) = NaN;

plot3(x,y,z,'bo'); hold on; plot3(xx,yy,zz,'b-'); hold off; grid
xlabel('n'); 
ylabel('real(s)'); 
zlabel('imag(s)');