title: find the angle of point related to pivot
body: <p>It’s been days that I’m working on a project which I have to use convex hull in it, and specific method of graham scan. The problem has been solved till the place I want to sort the points.
  So the story is that I have collected bunch of points which they are from the type Point and their coordinates are relatives. Mean they come from the mouse event x and y. So I have collected the mouse positions as x and y of the points. And I want to find the angle related to the pivot point. Does anyone have a piece of code to calculate that angle?
  Many and many thanks, the image below is what I need:
  <img src="https://i.stack.imgur.com/EUHRs.jpg" alt="Angles over pivote"></p>