Is there a way to apply FFT to an unevenly spaced time series?
You can try creating a new evenly spaced set of samples, Y', by interpolation.  Then FFT Y'.  How well this works depends on well the form of interpolation you choose matches the physics that produced your original Y,X data.
i have two series X and Y. X contains time values and Y contains a real function values for those times. I wish to perform FFT of the Y signal in python. The problem is that X is unevenly spaced: X(n+1)-X(n) /= constant. Is there a known way to apply FFT to such data ?