PRO chsign,x,y
;+
; This little routine finds sign changes in the y array and prints
; out the x and y values where the sign changes occur.
;-
sign=fltarr(n_elements(y))-1
sign(where(y GT 0))=1
shsign=shift(sign,1)
cross=WHERE(sign*shsign EQ -1)
prsc,x(cross)
prsc,y(cross)
RETURN
END