I am not certain exactly what you want.
See if this works for you:
x = sort(rand(1, 20)); % Create Data
y = sort(rand(1, 20)); % Create Data
x_5 = x(1:5);
y_5 = y(1:5);
b = [ones(size(x_5(:))), x_5(:)]\y_5(:); % Estimate Parameters
y_int = b(1); % Y-Intercept
y_xtrp = [ones(size([0; x_5(:)])), [0; x_5(:)]]*b; % Create Line
figure(1)
plot(x, y, 'bp')
hold on
plot([0; x_5(:)]', y_xtrp', '--r')
hold off
grid
