Help: x11 position in the Unix environment

I don't believe so. In general, under Unix/Linux, the Window Manager
determines window positioning upon startup unless the application
overrides this behavior. Some applications let you specify application
window positioning via command line 'geometry' arguments or via the use
of an .Xresources file. 

Splus used to have extensive control of the graphics window via X 
resources - here's a chunk from an old Xresources file of mine:

splus*background:       blue
splus*Canvas.height:    632
splus*Canvas.width:     800
splus*Command*background:       red
splus*Command*foreground:       cyan
splus*Label*background: cyan
splus*Label*foreground: red
splus*Text*background:  #a0f
splus*Text*foreground:  yellow
splus*colors:   white black white 54 black

  - and so on and so forth. Sadly I dont think such customisation is 
coded into R's X11 graphics window. Looking at the code there is no sign 
of it playing with X11's resource database. Shame.

Baz