15.37.9  EXPONENTIAL 

Does a fit to:
     Y=EXP( a0+a1*(x-offset)) + Polynomial when NONLINEAR=ON (Default)
          or ...
     LN(Y)=series(x-offset)
for data points with Y>0.0.
Where series is either POLY,SINE,COSINE or LEGENDRE.
If  you  wish to fit data which has negative or zero data points, you must
either SMOOTH the data to get rid of them, or use the BIN command to rebin
the  data  into large bins.  If all y data is less than zero, you may make
it positive with the command:
     TD:Y=Y TIMES -1

When  a NONLINEAR fit is used, and NTERMS=n is greater than 2 a polynomial
background of n-2 terms is added to the exponential.