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copied!<p>If I understand your question correctly, you want something like that:</p> <pre><code>>> p = polyfit([0 0.7],[0.1 1],1) p = 1.2857 0.1 >> figure >> hold on >> plot(x(y>=p(1)*x+p(2)),y(y>=p(1)*x+p(2)),'k.','MarkerSize',8) >> plot(x(y<=p(1)*x+p(2)),y(y<=p(1)*x+p(2)),'r.','MarkerSize',8) >> plot([0 0.7],[0.1 1],'g-') >> grid on,xlabel('x_1'),ylabel('x_2') </code></pre> <p>This is what I get in Octave:</p> <p><img src="https://i.stack.imgur.com/Uaaaq.jpg" alt="enter image description here"></p> <p><strong>EDIT following discussion in comments</strong>:</p> <p>There's not much more simplifying yo do, other than the following to make it more readable and debuggable in the future:</p> <pre><code>x = rand(500,1); y = rand(500,1); p = polyfit([0 0.7],[0.1 1],1); p2 = polyfit([0.4 1],[1 0],1); p3 = polyfit([0 1],[0.3 0.6],1); figure hold on plot(x,y,'m.','MarkerSize',8) idx1 = y>=polyval(p,x) & y<=polyval(p2,x) & y>=polyval(p3,x); idx2 = y>=polyval(p2,x) & y<=polyval(p,x) & y>=polyval(p3,x); idx3 = y<=polyval(p3,x) & y<=polyval(p,x) & y<=polyval(p2,x); plot(x(idx1),y(idx1),'y.','MarkerSize',8) plot(x(idx2),y(idx2),'r.','MarkerSize',8) plot(x(idx3),y(idx3),'g.','MarkerSize',8) plot([0 0.7],[0.1 1],'k-') plot([0.4 1],[1 0],'k-') plot([0 1],[0.3 0.6],'k-') grid on,xlabel('x_1'),ylabel('x_2') </code></pre> <p>This results in the following figure:</p> <p><img src="https://i.stack.imgur.com/owOby.jpg" alt="enter image description here"></p>

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