errorbarlogx.m

function errorbarlogx(epsilon)
%ERRORBARLOGX Homogenize the error bars for X-axis in log scale.
%   ERRORBARLOGX turns the X-axis of the current error bar plot to log
%   scale, and homogonizes the length of the horizontal segements which
%   terminate the vertical error bars.
%
%   By default, Matlab's ERRORBAR draws vertical error bars which are
%   terminated by small horizontal segments of uniform length for the X-
%   axis in linear scale. But when turning the X-axis to log scale, these
%   segments become uneven. Using ERRORBARLOGX makes them uniform again.
%
%   ERRORBARLOGX(N) specifies the relative length of the horizontal
%   segments, normalized with the total range of the data. By default,
%   N=0.01 is used.
%
%   Limitations: ERRORBARLOGX acts only on the last drawn curve. If this
%   curve is not an error bar plot, it won't work.
%
%   Example:
%      x=logspace(1,3,20);
%      y=5*(1 + 0.5*(rand(1,20)-0.5)).*x.^(-2);
%      errorbar(x,y,y/2,'o-');
%      errorbarlogx(0.03);
%       
%   F. Moisy
%   Revision: 1.00,  Date: 2006/01/20
%
%   See also ERRORBAR.


% History:
% 2006/01/20: v1.00, first version.

if nargin==0, epsilon=0.01; end; % default normalized segment length
 
set(gca,'XScale','log'); % set the X axis in log scale.

ca = get(gca); % current axe properties
cd = get(ca.Children(1)); % current data properties

heb = cd.Children(2); % handle to current error bars
ceb = get(heb); % current error bars properties

% ceb.XData is an array of length 9*length(data).
% for each data point, ceb.XData contains 3 blocks of 3 numbers:
% X0 X0 NaN X0-DX X0+DX Nan XO-DX X0+DX Nan
% So it is necessary to change the 4th, 5th, 7th and 8th values.

% logarithmic length of the horizontal segments:
dx = 10^(log10(ceb.XData(length(ceb.XData)-8)/ceb.XData(1))*epsilon);

% computes the new horizontal segments for each data point:
for i=1:(length(ceb.XData)/9),
    ii=(i-1)*9+1; % index of the first error bar data for the point #i
    ceb.XData(ii+3) = ceb.XData(ii)/dx;
    ceb.XData(ii+4) = ceb.XData(ii)*dx;
    ceb.XData(ii+6) = ceb.XData(ii)/dx;
    ceb.XData(ii+7) = ceb.XData(ii)*dx;
end;

set(heb,'XData',ceb.XData); % sets the changes in the error bar properties