title: Change x axis to scale with pi and not numbers
body: <p>I want to create the plot of cos(x) and sin(x) in Octave. I have learned how to change things such as xlabel  and xlim, however I can’t find how to change from having numbers (e.g 1, 2, 3) to having pi terms (-pi,-pi/2 and such). I would also appreciate if you can explain me how to do that. Here is a picture of what I want to do in case my english confused you.</p>
  <p><img src="https://i.stack.imgur.com/nq5nO.png" alt="enter image description here" /></p>