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+## Copyright (C) 1995-2012 Kurt Hornik
+##
+## This file is part of Octave.
+##
+## Octave 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 3 of the License, or (at
+## your option) any later version.
+##
+## Octave 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 Octave; see the file COPYING.  If not, see
+## <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {[@var{p}, @var{y}] =} ppplot (@var{x}, @var{dist}, @var{params})
+## Perform a PP-plot (probability plot).
+##
+## If F is the CDF of the distribution @var{dist} with parameters
+## @var{params} and @var{x} a sample vector of length @var{n}, the
+## PP-plot graphs ordinate @var{y}(@var{i}) = F (@var{i}-th largest
+## element of @var{x}) versus abscissa @var{p}(@var{i}) = (@var{i} -
+## 0.5)/@var{n}.  If the sample comes from F, the pairs will
+## approximately follow a straight line.
+##
+## The default for @var{dist} is the standard normal distribution.  The
+## optional argument @var{params} contains a list of parameters of
+## @var{dist}.  For example, for a probability plot of the uniform
+## distribution on [2,4] and @var{x}, use
+##
+## @example
+## ppplot (x, "uniform", 2, 4)
+## @end example
+##
+## @noindent
+## @var{dist} can be any string for which a function @var{dist_cdf}
+## that calculates the CDF of distribution @var{dist} exists.
+##
+## If no output arguments are given, the data are plotted directly.
+## @end deftypefn
+
+## Author: KH <Kurt.Hornik@wu-wien.ac.at>
+## Description: Perform a PP-plot (probability plot)
+
+function [p, y] = ppplot (x, dist, varargin)
+
+  if (nargin < 1)
+    print_usage ();
+  endif
+
+  if (! isvector (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 = str2func (sprintf ("%scdf", dist));
+  endif;
+  if (nargin <= 2)
+    y = feval (F, s);
+  else
+    y = feval (F, s, varargin{:});
+  endif
+
+  if (nargout == 0)
+    plot (p, y);
+    axis ([0, 1, 0, 1]);
+  endif
+
+endfunction
+
+
+%% Test input validation
+%!error ppplot ();
+%!error ppplot (ones(2,2));
+