q = 6;  # hardcoded

Z = phi(X, q);
w = linear_regression(Z, y);

# Define the ranges of the grid
u = linspace(-20, 30, 100);
v = linspace(-20, 30, 100);

# Initialize space for the values to be plotted
z = zeros(length(u), length(v));

for i = 1:length(u)
    for j = 1:length(v)
        # Notice the order of j, i here!
        z(j, i) = map_feature(u(i), v(j), q) * w;
    end
end

x1 = X(:, 2);
x2 = X(:, 3);
plot(x1, x2, '.')
hold on
contour(u,v,z, [0, 0], 'LineWidth', 2)
hold off