function getAngle(p1, p2, props) {
  if (!props) {
    props = PROPS_XY;
  }
  var x = p2[props[0]] - p1[props[0]];
  var y = p2[props[1]] - p1[props[1]];
  return Math.atan2(y, x) * 180 / Math.PI;
}
@private
calculate the angle between two coordinates
@param {Object} p1
@param {Object} p2
@param {Array} [props] containing x and y keys
@return {Number} angle