15.35  FFT 

FFT [SETS=[FROM] n1 [TO] [n2]] [SELECT="name"] [CHECK[=ON|OFF]] [LOG[=ON|
          OFF]] [INVERT[=ON|OFF]] [APPEND[=ON|OFF]] [NAME="name"]
          [POLAR[=ON|OFF]] [MONITOR[=ON|OFF]]
This command produces a complex fast fourier transformation of the
selected data sets.  For 2-d data the real part of the coefficients is
stored in Y, and the imaginary part in Z if it exists.  If not then DY
contains the imaginary part.  X contains the frequency value.  No error
analysis is performed by this operation.  For mesh data the real part is
stored in Z and the imaginary part in DZ.
     1.  APPEND=ON Creates a new data set containing the result.
     2.  CHECK=OFF Turns off the checking to see if data is a histogram.
     3.  INVERT=ON inverts the forier series contained in Y,Z/DY.
     4.  SETS selects the data sets to to transform.
     5.  LOG=ON Logs the operation on your terminal.
     6.  MONITOR If selected it histograms the original data set and the
         result.
         See:  SET MODE MONITOR
     7.  POLAR=ON selects polar coordinate output where Y is the magnitude
         and Z/DY is the angle.  The angle units is selected by the SET
         POLAR command.

The selected data must be a histogram, that is the X values must be
equally spaced and strictly monotonic, and there must be at least 2 points
in each data set.