Processing of data in colums is specified by one of the following letters:

 S specifies smooth lines (parametric cubic splines) which either go through the data points (default)
   or form some "line to guide the eye" by specifying the display parameter WEIGHT > 0.
   In the latter case the errors of the data points (if available) are multiplied
   by the specified weight factor in order to compute weighted splines.
   The larger the errors or the weight factors the smoother the resulting spline curves.
   If the coordinates of a data point appear twice the data are split at that point and
   smoothing is done separately for each group of coordinates

 F fit a polynomial to the data, draw the fit curve over the whole picture.

FF fit a polynomial to the data, draw the fit curve only between data points.

The order of the fitting polynomial is controlled by the display parameter POLYNOMIAL.
Data columns assigned an F or FF may also be fitted with the separate fit package
by specifing a polynomial order < 0.