  plbox(xopt, xtick, nxsub, yopt, ytick, nysub)
  *********************************************

  plbox

  Draws a box around the currently defined viewport, and labels it with
  world coordinate values appropriate to the window. Thus plbox should only
  be called after defining both viewport and window. The character strings
  xopt and yopt specify how the box should be drawn as described below. If
  ticks and/or subticks are to be drawn for a particular axis, the tick
  intervals and number of subintervals may be specified explicitly, or they
  may be defaulted by setting the appropriate arguments to zero.

  xopt (char *, input): Pointer to character string specifying options for
  horizontal axis. The string can include any combination of the following
  letters (upper or lower case) in any order:

   o a: Draws axis, X-axis is horizontal line y=0, and Y-axis is vertical line
     x=0.
   o b: Draws bottom (X) or left (Y) edge of frame.
   o c: Draws top (X) or right (Y) edge of frame.
   o g: Draws a grid at the major tick interval.
   o i: Inverts tick marks, so they are drawn outwards, rather than inwards.
   o l: Labels axis logarithmically. This only affects the labels, not the
     data, and so it is necessary to compute the logarithms of data points
     before passing them to any of the drawing routines.
   o m: Writes numeric labels at major tick intervals in the unconventional
     location (above box for X, right of box for Y).
   o n: Writes numeric labels at major tick intervals in the conventional
     location (below box for X, left of box for Y).
   o s: Enables subticks between major ticks, only valid if t is also
     specified.
   o t: Draws major ticks.

  xtick (PLFLT, input): World coordinate interval between major ticks on
  the x axis. If it is set to zero, PLplot automatically generates a suitable
  tick interval.

  nxsub (PLINT, input): Number of subintervals between major x axis ticks
  for minor ticks. If it is set to zero, PLplot automatically generates a
  suitable minor tick interval.

  yopt (char *, input): Pointer to character string specifying options for
  vertical axis. The string can include any combination of the letters
  defined above for xopt, and in addition may contain:

   o v: Write numeric labels for vertical axis parallel to the base of the
     graph, rather than parallel to the axis.

  ytick (real, input): World coordinate interval between major ticks on
  the y axis. If it is set to zero, PLplot automatically generates a suitable
  tick interval.

  nysub (PLINT, input): Number of subintervals between major y axis ticks
  for minor ticks. If it is set to zero, PLplot automatically generates a
  suitable minor tick interval.

