centeraxes(ax,opt)

function out = centeraxes(ax,opt)
% out = centeraxes(ax,opt)
%==========================================================================
% Center the coordinate axes of a plot so that they pass through the 
% origin.
%
% Input: 0, 1 or 2 arguments
%        0: Moves the coordinate axes of 'gca', i.e. the currently active 
%           axes.
%        1: ax, a handle to the axes which should be moved.
%        2: ax, opt, where opt is a struct with further options
%                opt.fontsize = 'size of axis tick labels'
%                opt.fontname = name of axis tick font.
%
% If the opt struct is ommitted, the current values for the axes are used.
%
% Output: stuct out containing handles to all line objects created by this
%         function.
%
%==========================================================================
% Version: 1.1
% Created: October 1, 2008, by Johan E. Carlson
% Last modified: November 21, 2009, by Johan E. Carlson
%==========================================================================

if nargin < 2,
    fontsize = get(ax,'FontSize');
    fontname = get(ax,'FontName');
end
if nargin < 1,
    ax = gca;
end

if nargin == 2,
    if isfield(opt,'fontsize'),
        fontsize = opt.fontsize;
    else
        fontsize = get(ax,'FontSize');
    end;
    if isfield(opt,'fontname'),
        fontname = opt.fontname;
    else
        fontname = get(ax,'FontName');
    end
end

axes(ax);
set(gcf,'color',[1 1 1]);
xtext = get(get(ax,'xlabel'),'string');
ytext = get(get(ax,'ylabel'),'string');

%--------------------------------------------------------------------------
% Check if the current coordinate system include the origin. If not, change
% it so that it does!
%--------------------------------------------------------------------------
xlim = 1.1*get(ax,'xlim');
ylim = 1.1*get(ax,'ylim');
set(ax,'xlim',xlim);
set(ax,'ylim',ylim);

if xlim(1)>0,
    xlim(1) = 0;
end

if ylim(1)>0,
    ylim(1) = 0;
end

if xlim(2) < 0,
    xlim(2) = 0;
end

if ylim(2) < 0,
    ylim(2) = 0;
end;

set(ax,'xlim',xlim,'ylim',ylim);


% -------------------------------------------------------------------------
% Caculate size of the "axis tick marks"
% -------------------------------------------------------------------------
axpos = get(ax,'position');
figpos = get(gcf,'position');
aspectratio = axpos(4) / (axpos(3));
xsize = xlim(2) - xlim(1);
ysize = ylim(2) - ylim(1);
xticksize = ysize/figpos(4)*12;
yticksize = xsize*aspectratio/figpos(3)*12;

% -------------------------------------------------------------------------
% Store old tick values and tick labels
% -------------------------------------------------------------------------
ytick = get(ax,'YTick');
xtick = get(ax,'XTick');
xticklab = get(ax,'XTickLabel');
yticklab = get(ax,'YTickLabel');

% -------------------------------------------------------------------------
% Draw new coordinate system
% -------------------------------------------------------------------------

yax = line([0; 0],[ylim(1)-1; ylim(2)]);
xax = line([xlim(1); xlim(2)],[0; 0]);
set(xax,'color',[0 0 0])
set(yax,'color',[0 0 0])

% Draw x-axis ticks
for k = 1:length(xtick),
    newxtick(k) = line([xtick(k); xtick(k)],[-xticksize/2; xticksize/2]);
    if (xtick(k)~=0),
        newxticklab(k) = text(xtick(k),-1.5*xticksize, strtrim(xticklab(k,:)));
        set(newxticklab(k),'HorizontalAlignment','center',...
            'Fontsize',fontsize,'FontName',fontname);
    end
end
set(newxtick,'color',[0 0 0]);

% Draw y-axis ticks
for k = 1:length(ytick),
    newytick(k) = line([-yticksize/2; yticksize/2],[ytick(k); ytick(k)]);
    if (ytick(k)~=0),
        newyticklab(k) = text(-.8*yticksize,ytick(k), yticklab(k,:));
        set(newyticklab(k),'HorizontalAlignment','right',...
            'FontSize',fontsize,'FontName',fontname);
    end
end
set(newytick,'color',[0 0 0]);

%--------------------------------------------------------------------------
% Move xlabels
%--------------------------------------------------------------------------
newxlabel = text(xlim(2),-1.5*xticksize,xtext);
set(newxlabel,'HorizontalAlignment','center',...
    'FontWeight','demi','FontSize',fontsize+2,'FontName',fontname);

newylabel = text(-yticksize,ylim(2)*1.02,ytext);
set(newylabel,'HorizontalAlignment','right','VerticalAlignment','top',...
    'FontWeight','demi','FontSize',fontsize+2,'FontName',fontname);

%--------------------------------------------------------------------------
% Create arrowheads
%--------------------------------------------------------------------------
x = [0; -yticksize/4; yticksize/4];
y = [ylim(2); ylim(2)-xticksize; ylim(2)-xticksize];
patch(x,y,[0 0 0])

x = [xlim(2); xlim(2)-yticksize; xlim(2)-yticksize];
y = [0; xticksize/4; -xticksize/4];
patch(x,y,[0 0 0])

axis off;
box off;

%--------------------------------------------------------------------------
% Create output struct
%--------------------------------------------------------------------------

if nargout > 0,
    out.xaxis = xax;
    out.yaxis = yax;
    out.xtick = newxtick;
    out.ytick = newytick;
    out.xticklabel = newxticklab;
    out.yticklabel = newyticklab;
    out.newxlabel = newxlabel;
    out.newylabel = newylabel;
end;