You can do that approximately.
% this is the "finite difference" derivative. Note it is one element shorter than y and x
yd = diff(y)./diff(x);
% this is to assign yd an abscissa midway between two subsequent x
xd = (x(2:end)+x(1:(end-1)))/2;
% this should be a rough plot of your derivative
plot(xd,yd)
Of course the reliability of the above procedure depends on the thickness of your grid x as well as on the properties of y.
You could perhaps do a slightly (cosmetically) better job by interpolating your data using interp1.
