How to add legend in a highlighted graph? - matlab

Question

I want to add legend in a graph G according to different highlighted edges. Is it possible to do it with only one graph G?
Here is a toy example to play with. I have a plot G.
adj =[0 0 1 1 1; % adjacency matrix
1 0 1 0 1;
0 1 0 1 1;
1 1 1 0 1;
0 0 1 0 0]
G = digraph(adj);
I highlighted all edges with 3 colors according to types of edges. 3 types of edges indicate there are 3 different relation between nodes in my case.
This is how I highlighted all edges:
M(:,:,1)=[0 0 1 0 0;1 0 0 0 1;0 0 0 0 0;1 0 0 0 0;0 0 1 0 0];
M(:,:,2)=[0 0 0 1 0; 0 0 1 0 0;0 1 0 0 1;0 0 0 0 0;0 0 0 0 0];
M(:,:,3)=[0 0 0 0 1; 0 0 0 0 0; 0 0 0 1 0;0 1 1 0 1;0 0 0 0 0];
The difficulty in my problem is that I have to remove vertices whose out-degree is less than some integel (say it's 2). Thus I can't plot 3 graphs independently.
rmvNode=find(outdegree(G)<2); % outdegree is the reason why single G is neccesary
adj(rmvNode,:)=[]; adj(:,rmvNode)=[];
M(:,rmvNode,:)=[]; M(rmvNode,:,:)=[];
G=digraph(adj);
Then we can plot it.
for k=1:3 %Looping depending on the third dimension
[r,c]= find(M(:,:,k)); %Finding non-zero elements
s{k}=r; t{k}=c;
end
h=plot(G);
highlight(h,s{1},t{1},'EdgeColor','r');
highlight(h,s{2},t{2},'EdgeColor','g');
highlight(h,s{3},t{3},'EdgeColor','b');
My ideal situation would be a legend like this: assign red edges to label 'type 1', assign blue edges to 'type 2', and assign green ones to 'type 3'. I want something like this:
Once more: I can't plot 3 graphs independently according to 3 pages in M, combine 3 plots together and then add a legend. Because as you can see, outdegree requires a whole graph G as input, it's not viable to divide G into G1, G2 and G3.

Answer 1

One way would be to manipulate the legend function by adding an invisible plot like this:
%put this at the end of your code
hold on; %to retain current plot
ax=plot(NaN,NaN,'r',NaN,NaN,'g',NaN,NaN,'b'); %plotting invisible points of desired colors
legend(ax,'Type 1','Type 2','Type 3'); %adding the legend
which gives: