Use the gradient function, not diff to calculate the derivative. It is the same size as the argument vector. This assumes that your ‘x’ values are evenly-spaced. See the documentation for details.
If the ‘x’ vector are not evenly-spaced (or acceptably close), consider using the polyfit, polyder, and polyval functions. The documentation for those functions will tell you how to use them. Use the lowest-order polynomial that will provide an acceptable fit to your data. Usually you can do this with at most a 7th-order polynomial.
