function d=derivXY(x,y)
% returns first derivative of y 
% with respect to x
% using 2-point central difference.
%  T. C. O'Haver, 1990.
n=length(x);
d(1)=y(2)-y(1)./(x(2)-x(1));
d(n)=y(n)-y(n-1)./(x(n)-x(n-1));
for j = 2:n-1;
  d(j)=(y(j+1)-y(j-1)) ./ (x(j+1)-x(j-1));
end