  plot3d(x, y, z, nx, ny, opt, side)
  **********************************

  plot3d

  Plots a three dimensional surface plot within the environment set up by
  plw3d. The surface is defined by the two-dimensional array z  , the point
  z[i][j] being the value of the function at (x[i], y[j]). Note that the
  points in arrays x and y do not need to be equally spaced, but must be
  stored in ascending order. The parameter opt controls the way in which the
  surface is displayed. For further details 3-d Plots.

  x (PLFLT *, input): Pointer to set of x coordinate values at which the
  function is evaluated.

  y (PLFLT *, input): Pointer to set of y coordinate values at which the
  function is evaluated.

  z (PLFLT **, input): Pointer to a vectored two-dimensional array with
  set of function values.

  nx (PLINT, input): Number of x values at which function is evaluated.

  ny (PLINT, input): Number of y values at which function is evaluated.

  opt (PLINT, input): Determines the way in which the surface is
  represented:

   o 1: Lines are drawn showing z as a function of x for each value of y[j].
   o 2: Lines are drawn showing z as a function of y for each value of x[i].
   o 3: Network of lines is drawn connecting points at which function is
     defined.

  side (PLINT, input): Flag to indicate whether or not ``sides'' should be
  draw on the figure. If side=0 no sides are drawn, otherwise the sides are
  drawn.

  mx (PLINT, input): Length of array in x direction, for plotting
  subarrays (Fortran only).

