Initialization of a graphics driver
xinit(FileName)
xinit()
To export to a file, FileName must be specified. It is the name of the file where all the graphics operations are recorded.
For screen drivers (X11 or Rec), xinit should be called without any argument and it will open an empty graphic window.
`driver`_("Pos")
xinit(TMPDIR+"/foo.ps")
`plot2d`_()
`xend`_()
`driver`_("X11")
`driver`_("PNG")
xinit(TMPDIR+"/foo.png")
`plot3d`_()
`xend`_()
`driver`_("X11")