Offset term calculated by FITLINE for data identified with a matching
'idexpr' value.  'idexpr' must be an integer from 0 to 9 and must
match some 'idexpr' value used in a previous FITLINE procedure
statement.  See FITLINE procedure description.

Use the SLOPE and OFFSET values to display and/or make use of the
line fit to the data by the FITLINE procedure.  DAN allows up to
ten simultaneously active lines and the 'idexpr' value is used to
distinguish which line your algorithm is referencing; hence, the same
'idexpr' value must be used when calling FITLINE, SLOPE, and OFFSET. 
+
For example, to fit a line to the data in file "fitdata" and then
plot both the original data and the line on the same graph, the
following algorithm could be used:

     SCALEY(fitdata) &
     FITLINE (1,fitdata);      {do linear least sqs fit to 'fitdata'}
          {plot original data and line fit to data }
     TRACE = BOX & PLOT(fitdata) &
     TRACE = LINE & PLOT( SLOPE(1)*X + OFFSET(1) );

A measure of how well the line fits the data can be determined by
adding the following lines to the above algorithm:

     ERASE;      {erase prev plot so new scaling can be used}
     residuals = SLOPE(1)*X + OFFSET(1) - fitdata &
     SCALEY(residuals);
     PLOT (residuals) & PLOT ( STD.DEV (residuals) );
