  plbin(nbin, x, y, cen)
  **********************

  plbin

  Plots a histogram consisting of n bins. The value associated with the
  i'th bin is placed in x[i], and the number of points in the bin is placed
  in y[i]. For proper operation, the values in x[i] must form a strictly
  increasing sequence. If center is false, x[i] is the left-hand edge of the
  i'th bin, and if center is true, the bin boundaries are placed midway
  between the values in the x array. Also see plhist for drawing histograms
  from unbinned data.

  nbin (PLINT, input): Number of bins (i.e., number of values in _x and _y
  arrays.)

  x (PLFLT *, input): Pointer to array containing values associated with
  bins. These must form a strictly increasing sequence.

  y (PLFLT *, input): Pointer to array containing number of points in bin.
  This is a PLFLT (instead of PLINT) array so as to allow histograms of
  probabilities, etc.

  cen (PLINT, input): Indicates whether the values in x represent the
  lower bin boundaries (cen=0) or whether the bin boundaries are to be midway
  between the x values (cen=1). If the values in x are equally spaced and
  cen=1, the values in x are the center values of the bins.

