/*****************************************************************************
*                                                                            *
*   TREND.SPL    Copyright (C) 1998 DSP Development Corporation              *
*                               All Rights Reserved                          *
*                                                                            *
*   Author:      Randy Race                                                  *
*                                                                            *
*   Synopsis:    Fits a line to a series                                     *
*                                                                            *
*   Revisions:    2 Jul 1998  RRR  Creation                                  *
*                23 Oct 1998  RRR  Polyfit does not create spec file         *
*                                                                            *
*****************************************************************************/

#include 

#if @HELP_TREND

    TREND

    Purpose: Fits a line to a series

    Syntax:  TREND(series)

              series - a series, the input data

    Returns: A series

    Example:
             W1: Integ(gnorm(1000,1))
             W2: Trend(w1);overp(w1, lred)

             W2 contains the least squares best linear fit of the data.
             The resulting line is plotted with the original data.

             W3: Xy(w1, deriv(w1))
             W4: Trend(w3);overp(w3, lred)

             W4 contains the best linear fit to the XY data in W3.

    Remarks:
             (fit, coeff) = trend(s) returns both the resulting fit and
             the linear coefficients {a0, a1} of the equation

                              y = a0 + a1 * x

    See Also:
             Polyfit
             Polygraph
#endif


trend(s)
{
        local coef, fit, nr, nc;

        /* linear fit coefficients */
        coef = polyfit(s, 1, -1);

        /* generate fitted data */
        fit = polygraph(coef, xvals(s), ISXYSERIES(s));

        /* return fit and coefficients */
        if (outargc > 1)
        {
                return(fit, coef);
        }
        else
        {
                return(fit);
        }
}