Add line(s) to 2D plots with specified colors

% ADDLINE_Z adds solid color line(s) in the axes passed by handle h_ax
% both 'value' and 'color' can be an array
% 'color' is set to 'k' if not given
%
% Syntax:
% addline_z(h_ax,xy,value,color); %% tested on MATLAB 7.9.0 (R2009b)
% e.g.:
% figure;
% x=[0:0.01:10];
% y=sin(x);
% plot(x,y);
% addline_z(get(gcf,'Children'),'y',[0.2,0.5,0.8],{'r','g','b'});
%
% Description:
% Too simple to be stated here.
%
% Programmed and Copyright by J. Zhang: xyzhangj@physics.ucla.edu
% $Version=0.1 $Date=2010/09/07