title: Gnu Octave Quadratic Equation Graphic
body: <p>I lost two days of trying to do this but with no result. How can I plot the quadratic equation's parabola and roots. Something like <a href="http://www.algebra.com/algebra/homework/quadratic/quadratic_equation.gif" rel="nofollow noreferrer">this</a>. I just need to be able to see the parabola and that it crosses the abscissa at the write coordinates.</p>
  
  <p>Here is what I have:</p>
  
  <pre><code>x = linspace(-50,50);
  y = 1.*x.*x - 8.*x + 15;
  plot(x,y)
  hold on;
  grid()
  
  rts = roots([1,-8,15]);
  plot(rts, zeros(size(rts)), 'o', "color", "r")
  </code></pre>
  
  <p>And the result is:
  <img src="https://i.stack.imgur.com/Gd2hY.jpg" alt="enter image description here"></p>
  
  <p>As you can see, the top of the parabola at 0 ordinate, instead of under it. I will appreciate your help!</p>