;+
; NAME:
;       FORCEFIT
; PURPOSE:
;       Force a fit in either slope or zero-point of a line
; CALLING SEQUENCE:
;       result = FORCEFIT([x, y[,a=a][,b=b]][/help])
; EXAMPLES:
;       x = dindgen(100)
;       y = dindgen(100)
;       result = FORCEFIT(x, y, a=2)
;           fits a line of slope 2 to the input values
;       result = FORCEFIT(x, y)
;           normal linear fit using the LINFIT
; KEYWORDS:
;       x = the input x values
;       y = the input y values
;       a = the fixed slope
;       b = the fixed zero-point
;       /help = displays this help
; MODIFICATION HISTORY:
;       before 27/08/04 written by Eduard Westra
;       27/08/04 added help header by Eduard Westra
;       31/10/06 added the case for b being set
;-
FUNCTION forcefit, x, y, a=a, b=b, help=help
IF (KEYWORD_SET(HELP)) THEN BEGIN & doc_library,'forcefit' & RETURN, [0.,0.] & ENDIF

    IF (N_ELEMENTS(a) EQ 1 AND N_ELEMENTS(b) EQ 1) THEN BEGIN
        PRINT, "You already set both parameters. What do you want me to do?!?"
        RETURN, [b,a]
    ENDIF
    IF N_ELEMENTS(a) EQ 1 THEN BEGIN
        div = a*x
        div = y - div
        RETURN, [TOTAL(div)/N_ELEMENTS(div),a]
    ENDIF
    IF N_ELEMENTS(b) EQ 1 THEN BEGIN
        ; implement later
       a = TOTAL(y)/TOTAL(x)
       ;a = TOTAL(x*y)/TOTAL(x*x)
       RETURN, [b, a]
    ENDIF
        ; no a and b set
    RETURN, LINFIT(x, y)
END