  plvpor(xmin, xmax, ymin, ymax)
  ******************************

  plvpor

  Device-independent routine for setting up the viewport. This defines the
  viewport in terms of normalized subpage coordinates which run from 0.0 to
  1.0 (left to right and bottom to top) along each edge of the current
  subpage. Use the alternate routine plsvpa in order to create a viewport of
  a definite size.

  xmin (PLFLT, input): The normalized subpage coordinate of the left-hand
  edge of the viewport.

  xmax (PLFLT, input): The normalized subpage coordinate of the right-hand
  edge of the viewport.

  ymin (PLFLT, input): The normalized subpage coordinate of the bottom
  edge of the viewport.

  ymax (PLFLT, input): The normalized subpage coordinate of the top edge
  of the viewport.

