Adding a legend manually for a plot generated by a loop

Hi,

I am generating a plot in Matlab one point at a time depending on how a condition is satisfied within a loop:

for i=1:size(Ind,1)
    if(Ind(i)==1)
        c='ro';
    elseif(Ind(i)==2)
        c='bo';
    elseif(Ind(i)==3) 
        c='go';
    end

     plot(i,Y(i),c) %plotting some other value with the color chosen.
     hold on
  end

How do I add a legend entry to this? I want to associate the index position(1,2 and 3) to red,blue and green in the legend.

Thanks!

Answer 1

[jgg]

I think the issue is that you don't want to generate the plot like that; it's slow and makes it hard to label. Check out this solution instead:

 Y = 10*rand(100,1); %your data
 Ind = rand(100,1) > 0.5;
 Ind = Ind + (rand(100,1) > 0.75);
 Ind = Ind + 1; %an index corresponding to the group/colour of Y
 Vals = [1:100]'; %the X-axis, or location of Y

 plot(Vals(Ind == 1),Y(Ind ==1), 'ro', Vals(Ind == 2), ...
 Y(Ind == 2), 'bo', Vals(Ind == 3), Y(Ind == 3), 'go')
 legend('Red Data', 'Blue Data', 'Green Data')

This is much more efficient and you don't have to loop over all your points.

So, in your example, you would just go:

 i = [1:size(Ind,1)];
 plot(i(Ind == 1),Y(Ind ==1), 'ro', i(Ind == 2), ...
 Y(Ind == 2), 'bo', i(Ind == 3), Y(Ind == 3), 'go')
 legend('Red Data', 'Blue Data', 'Green Data')

[Osh]

Thank you! That's exactly what I was looking for.