Excel's Fourier Analysis (FFT) Add-In is very useful, except that the user
must manually run it each time the input column's data changes in order to
see the new output data. Why not make this a function, so that FFT results
are updated automatically whenever the input is changed?

I'd love to see, for example, "FFT" and "IFFT" (inverse FFT) functions in a
future version of Excel.