Problems plotting and adding legend to certain point in a plot

Hi,

I have a simple code that plots 7 eigenvalues as follows:

A = load('XV15A_200kts_A9.txt');

Result = eig(A)

plot(Result,'+')

What I would like to do is for pairs of points (ie -2 + 0.5i and -2 - 0.5i) to have a different colour to other points and I would ideally like to label these points in the legend (so in this example i would like the two points to be marked red and in the legend say 'Dutch Roll Mode')

Any help would be appreciated

Answer 1

[Setsuna Yuuki.]

to place the dots with different colors:

compleja = 0;
A = rand(100);
result = eig(A)

j = 1;
for i = 1:length(result)-1
    if(result(i) == conj(result(i+1)))
       compleja(j) = result(i);
       j = j+1;
    end
    
end

result = result(2:length(result));
resultReal = real(result); resultComplex = imag(result);
scatter(resultReal,resultComplex,'g +','linewidth',2); hold on;
resultReal = real(compleja); resultComplex = imag(compleja);
scatter(resultReal,resultComplex,'r +','linewidth',2)
compleja = conj(compleja);
resultReal = real(compleja); resultComplex = imag(compleja);
scatter(resultReal,resultComplex,'b +','linewidth',2)
xlabel('Real'); ylabel('Imaginario')
legend('Only Real','Red category','Blue category')

A example with 100x100 matrix: