FUNCTION Get_Shift

;+
; NAME:
;	Get_Shift
; PURPOSE:
;	To know the value of the shift between plots selected.
; CATEGORY:
; 	Data Management
; CALLING SEQUENCE:
;	value = Get_Shift()
; COMMON BLOCK:
;	SelectionRetrieval, where this value is stored.
; -
; MODIFICATION HISTORY:
;	Created in June 1991 by A.Csillaghy

COMMON SelectionRetrieval, dum, shift

IF N_Elements(shift) EQ 0 THEN RETURN, 0.0 $
ELSE RETURN,shift

END