How can I define the period before fitting a Fourier series to discrete data?

I'm using MATLAB's fit function to fit a Fourier series to a set of discrete data. I need the period of the Fourier series to be 2*pi. However I can't work out how to fix this so that when I call fit(X,Y,'fourier8') it fits the series to my required period. Any suggestions would be greatly appreciated. Thanks.

[Rose]

I have the same problem. Did you find a solution? If not, would someone help us please?