title: Drawing arrowheads which follow the direction of the line in PyGame
body: <p>In Pygame, how can I calculate the coordinates for the three points of the head of an arrow, given a start point and an end point for the arrow, so that the arrowhead points in the same direction as the line?</p>
  
  <pre><code>def __draw_arrow(self, screen, colour, start, end):     
      start = self.__coordinate_lookup[start]
      end = self.__coordinate_lookup[end]
      dX = start[0] - end[0]
      dY = -(start[1] - end[1])
      print m.degrees(m.atan(dX/dY)) + 360 
      pygame.draw.line(screen,colour,start,end,2)
  </code></pre>
  
  <p>I've tried playing around with angles and with the gradient of the line, the fact the Y coordinates increase downwards instead of upwards is throwing me off and I would really appreciate a nudge in the right direction. </p>