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SMOOTH <data>,<series>,<smoothened_series>,<width> 
smooths <series> in <data> using a technique based on FFT
(Fast Fourier Transformation) with a window whose full width is of
order <width> neighboring points and saves the smoothened values as
<smoothened_series>.

SMOOTH can be used for smoothing any variable Y in relation to another
variable X by first sorting the data by X and then applying SMOOTH for
Y.
Numerical method:
SMOOTH removes any linear trend and uses then FFT to low-pass filter
the data. The linear trend is reinserted at the end.
<width> gives the "amount of smoothing", specified as the number of
points over which the data should be smoothened (not necessarily an
integer). Zero gives no smoothing at all. The results of SMOOTH are
generally in accord with the notion "draw a smooth curve through these
scattered points".
Reference: "Numerical Recipes" (1987) by Press, Flannery, Teukolsky, and
                                                              Vetterling
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