| grid.null {grid} | R Documentation | 
These functions create a NULL graphical object, which has zero width, zero height, and draw nothing. It can be used as a place-holder or as an invisible reference point for other drawing.
nullGrob(x = unit(0.5, "npc"), y = unit(0.5, "npc"),
         default.units = "npc",
         name = NULL, vp = NULL)
grid.null(...)
x | 
 A numeric vector or unit object specifying x-location.  | 
y | 
 A numeric vector or unit object specifying y-location.  | 
default.units | 
 A string indicating the default units to use
if   | 
name | 
 A character identifier.  | 
vp | 
 A Grid viewport object (or NULL).  | 
... | 
 Arguments passed to   | 
A null grob.
Paul Murrell
grid.newpage()
grid.null(name="ref")
grid.rect(height=grobHeight("ref"))
grid.segments(0, 0, grobX("ref", 0), grobY("ref", 0))