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

Astha Ameta

2017 1 28

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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