code: function getRotation(start, end) {
    return getAngle(end[1], end[0], PROPS_CLIENT_XY) - getAngle(start[1], start[0], PROPS_CLIENT_XY);
  }
docs: calculate the rotation degrees between two pointersets
  @param {Array} start array of pointers
  @param {Array} end array of pointers
  @return {Number} rotation