Plot Support Vectors, Margin and decision boundary using LIBSVM MATLAB interface

I am using Matlab-Libsvm Interface for binary classification using SVM. I have multi dimensional data. I need to plot decision boundary and margin along with support vectors.

Below is the code I used for this:

model = svmtrain(Y, X, sprintf('-c %f -w1 1 -w-1 5 -g %f', best_C, best_gamma));

w = model.SVs' * model.sv_coef;

b = -model.rho;

if (model.Label(1) == -1)

      w = -w; b = -b;

end

y_hat = sign(w'*X' + b);

sv = full(model.SVs);

% plot support vectors

figure

plot(sv(:,1:14),'ko', 'MarkerSize', 10);

hold on

% plot decision boundary

plot_x = linspace(min(X(:,1)), max(X(:,1)), 30);

plot_y = (-1/w(2))*(w(1)*plot_x + b);

plot_z = sign(w(1)*plot_x + b)/norm(w);

plot(plot_x, plot_y, 'k-', 'LineWidth', 1)

hold on

plot(plot_x, plot_z, 'kx', 'LineWidth', 1);

hold off

hold off

But I am unable to plot decision boundary and margin using this code. Also, support vectors are plotted inappropriately.

Please let me know where I am wrong.

[babita verma]

Hi Astha I am facing the same problem with plotting of the decision boundary. Were you able to resolve this issue?

regards