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

  plsvpa

  Alternate routine to plvpor for setting up the viewport. This routine
  should be used only if the viewport is required to have a definite size in
  millimeters. The routine plgspa is useful for finding out the size of the
  current subpage.

  xmin (PLFLT, input): The distance of the left-hand edge of the viewport
  from the left-hand edge of the subpage in millimeters.

  xmax (PLFLT, input): The distance of the right-hand edge of the viewport
  from the left-hand edge of the subpage in millimeters.

  ymin (PLFLT, input): The distance of the bottom edge of the viewport
  from the bottom edge of the subpage in millimeters.

  ymax (PLFLT, input): The distance of the top edge of the viewport from
  the top edge of the subpage in millimeters.

