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ppplot.m

## Copyright (C) 1995, 1996, 1997  Kurt Hornik
## 
## This program is free software; you can redistribute it and/or modify
## it under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 2, or (at your option)
## any later version.
## 
## This program is distributed in the hope that it will be useful, but
## WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
## General Public License for more details. 
## 
## You should have received a copy of the GNU General Public License
## along with this file.  If not, write to the Free Software Foundation,
## 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

## usage:  [p, y] = ppplot (x [, dist [, params]])
##
## Performs a PP-plot (probability plot).
##
## If F is the CDF of the distribution `dist' with parameters `params'
## and x a sample vector of length n, the PP-plot graphs ordinate y(i) =
## F (i-th largest element of x) versus abscissa p(i) = (i - 0.5)/n. If
## the sample comes from F, the pairs will approximately follow a
## straight line.
##
## The default for `dist' is the standard normal distribution.  The
## optional argument `params' contains a list of parameters of
## `dist'. E.g., for a probability plot of the uniform distribution on
## [2,4] ans x, use `ppplot (x, "uniform", 2, 4)'.
##
## If no output arguments are given, the data are plotted directly.

## Author:  KH <Kurt.Hornik@ci.tuwien.ac.at>
## Description:  Perform a PP-plot (probability plot)

function [p, y] = ppplot (x, dist, ...)
  
  if (nargin < 1)
    usage ("ppplot (x [, dist [, params]])");
  endif
  
  if !is_vector (x)
    error ("ppplot:  x must be a vector.");
  endif

  s = sort (x);
  n = length (x);
  p = ((1 : n)' - 0.5) / n;
  if (nargin == 1)
    F = "stdnormal_cdf";
  else
    F = sprintf ("%s_cdf", dist);
  endif;
  if (nargin <= 2)
    y = feval (F, s);
  else
    y = feval (F, s, all_va_args);
  endif
  
  if (nargout == 0)
    axis ([0, 1, 0, 1]);
    set nokey;
    plot (p, y);
  endif

endfunction

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