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1 ## Copyright (C) 1995, 1996, 1997 Kurt Hornik |
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2 ## |
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3 ## This file is part of Octave. |
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4 ## |
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5 ## Octave is free software; you can redistribute it and/or modify it |
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6 ## under the terms of the GNU General Public License as published by |
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7 ## the Free Software Foundation; either version 3 of the License, or (at |
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8 ## your option) any later version. |
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9 ## |
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10 ## Octave is distributed in the hope that it will be useful, but |
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11 ## WITHOUT ANY WARRANTY; without even the implied warranty of |
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12 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
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13 ## General Public License for more details. |
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14 ## |
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15 ## You should have received a copy of the GNU General Public License |
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16 ## along with Octave; see the file COPYING. If not, see |
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17 ## <http://www.gnu.org/licenses/>. |
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18 |
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19 ## -*- texinfo -*- |
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20 ## @deftypefn {Function File} {[@var{p}, @var{y}] =} ppplot (@var{x}, @var{dist}, @var{params}) |
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21 ## Perform a PP-plot (probability plot). |
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22 ## |
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23 ## If F is the CDF of the distribution @var{dist} with parameters |
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24 ## @var{params} and @var{x} a sample vector of length @var{n}, the |
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25 ## PP-plot graphs ordinate @var{y}(@var{i}) = F (@var{i}-th largest |
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26 ## element of @var{x}) versus abscissa @var{p}(@var{i}) = (@var{i} - |
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27 ## 0.5)/@var{n}. If the sample comes from F, the pairs will |
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28 ## approximately follow a straight line. |
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29 ## |
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30 ## The default for @var{dist} is the standard normal distribution. The |
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31 ## optional argument @var{params} contains a list of parameters of |
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32 ## @var{dist}. For example, for a probability plot of the uniform |
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33 ## distribution on [2,4] and @var{x}, use |
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34 ## |
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35 ## @example |
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36 ## ppplot (x, "uniform", 2, 4) |
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37 ## @end example |
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38 ## |
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39 ## @noindent |
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40 ## @var{dist} can be any string for which a function @var{dist_cdf} |
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41 ## that calculates the CDF of distribution @var{dist} exists. |
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42 ## |
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43 ## If no output arguments are given, the data are plotted directly. |
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44 ## @end deftypefn |
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45 |
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46 ## Author: KH <Kurt.Hornik@wu-wien.ac.at> |
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47 ## Description: Perform a PP-plot (probability plot) |
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48 |
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49 function [p, y] = ppplot (x, dist, varargin) |
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50 |
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51 if (nargin < 1) |
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52 print_usage (); |
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53 endif |
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54 |
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55 if (! isvector (x)) |
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56 error ("ppplot: x must be a vector"); |
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57 endif |
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58 |
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59 s = sort (x); |
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60 n = length (x); |
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61 p = ((1 : n)' - 0.5) / n; |
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62 if (nargin == 1) |
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63 F = @stdnormal_cdf; |
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64 else |
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65 F = str2func (sprintf ("%s_cdf", dist)); |
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66 endif; |
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67 if (nargin <= 2) |
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68 y = feval (F, s); |
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69 else |
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70 y = feval (F, s, varargin{:}); |
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71 endif |
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72 |
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73 if (nargout == 0) |
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74 plot (p, y); |
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75 axis ([0, 1, 0, 1]); |
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76 endif |
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77 |
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78 endfunction |
