  plcont(z, nx, ny, kx, lx, ky, ly, clevel, nlevel, pltr)
  *******************************************************

  plcont

  Draws a contour plot of the data in z[nx][ny], using the nlevel contour
  levels specified by clevel. Only the region of the array from kx to lx and
  from ky to ly is plotted out. A transformation routine pltr is used to map
  indicies within the array to the world coordinates. Contour plots for more
  information.

  z (PLFLT **, input): Pointer to a vectored two-dimensional array
  containing data to be contoured.

  nx, ny (PLINT, input): Physical dimensions of array z.

  kx, lx (PLINT, input): Range of x indicies to consider.

  ky, ly (PLINT, input): Range of y indicies to consider.

  clevel (PLFLT *, input): Pointer to array specifying levels at which to
  draw contours.

  nlevel (PLINT, input): Number of contour levels to draw.

  pltr (void *, input): Pointer to function that defines transformation
  between indicies in array z and the world coordinates (C only). Two
  transformation functions are provided: xform, for simple linear mappings,
  and pltr0, if no mapping is required. xform is not actually compiled into
  the PLplot library due to the need for external data; instead you must
  include it as part of your program. The function should have the form:

   pltr(x, y, tx, ty)

  x, y (PLFLT, input):  Specifies the position in the array through which
  the contour runs in terms of the array indicies.

  tx, ty (PLFLT *, output): Pointers to the world coordinates
  corresponding to the point (x, y).

  tr (PLFLT *, input): Pointer to 6-element array specifying coefficients
  to use in the xform transformation function (Fortran only). Since this is
  somewhat inflexible, the user is recommended to call either of plcon0,
  plcon1, or plcon2 instead.

