function intplot(trait, mark1, mark2, mnames)
%make an interaction plot
K = max(max([mark1, mark2]))+1;	%2 for BC, 3 for F2
means = zeros(K,K);
ns = zeros(K,K);
stds = zeros(K,K);
for i = 1:K
   for j = 1:K
      means(i,j) = mean(trait(find(mark1==i-1 & mark2==j-1)));
      ns(i,j) = length(trait(find(mark1==i-1 & mark2==j-1)));
      if ns(i,j) > 2
         stds(i,j) = std(trait(find(mark1==i-1 & mark2==j-1)));
      end
   end
end
stds = stds./sqrt(ns);
plot(means,'-d')
axis([0.5 K+0.5 .9*min(min(means-stds)), 1.1*max(max(means+stds))])
hold on
for i = 1:K
   for j = 1:K
      plot([i,i]',[means(i,j)-stds(i,j),means(i,j)+stds(i,j)]','+:')
   end
end
set(gca,'XTick',[1:K])
if K == 2
   set(gca,'XTickLabel',['AA';'AB'])
else
   set(gca,'XTickLabel',['AA';'AB';'BB'])
end
if nargin > 3
   xlabel(mnames(1))
end

text(K+0.25, means(K,1), 'AA')
text(K+0.25, means(K,2), 'AB')
if K == 3
   text(K+0.25, means(K,3), 'BB')
end
if nargin > 3
   gtext(mnames(2));
end

hold off