function addplot(x,y,n)
%addplot(x,y)
%Adds a line to a plot each time it is called, 
%using a new color and linestyle for each new line.
%(start by calling plot; then add new ones with addplot)
%
%addplot(x,y,n) uses n as the index for the plot--if n is incremented 
%each time, you get a new color each time.  The main point of this is that
%if you call it with n=1, it starts a new plot, rather than your
%needing to call a different function (plot) for the first one.
%
%Because there is a cycle of seven colors and four line styles, there are 28 different
%unique combinations of line styles and colors used before they start repeating.  

persistent index  %variable that is saved to keep track of which color was used last.
if isempty(index), index = 2;, end  %First time it is called, use second color

if nargin > 2, index = n; end  % Use index if it is given

colorindex=mod(index-1,7)+1;
lineindex = mod(index-1,4)+1;

colororder =  [0         0    1.0000
         0    0.5000         0
    1.0000         0         0
         0    0.500    0.500
    0.6500         0    0.6500
    0.5500    0.5500         0
    0.2500    0.2500    0.2500];

linestyleorder = {'-','--',':','-.'};

if index > 1,  %normal situation--want to turn on hold to add plot
    hold on
    plot(x,y,linestyleorder{lineindex},'color',colororder(colorindex,:) )
    hold off
else   %first time--want hold off
    hold off
    plot(x,y,linestyleorder{lineindex},'color',colororder(colorindex,:) )
end

index = index+1;  %increment index for next time.