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.
Usage
nullGrob(x = unit(0.5, "npc"), y = unit(0.5, "npc"),
default.units = "npc",
name = NULL, vp = NULL)
grid.null(...)
Arguments
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 x, y, width, or height
are only given as numeric vectors.