/EM> arguments.
<EM>:xcenter</EM> and <EM>:ycenter</EM> arguments determine
the physical location of the viewport.
These two parameters actually decide the location where objects are drawn
on the screen when you are using a primitive display device like tektronics
4014 on which every dimension must be given absolutely to the origin of the
screen.
If you are using more sophisticated display device like Xwindows where
locations can be determined relatively to the parent window, you need not
to change viewport