docs: function calculatePositionDiff(position) {
    if (!_lastDiffPosition) {
      return _lastDiffPosition = position;
    }
    var dx, dy, p1, p2;
    p1 = position;
    p2 = _lastDiffPosition;
    dx = p2.x - p1.x;
    dy = p2.y - p1.y;
    _calcMoveDiffX += dx;
    _calcMoveDiffY += dy;
    _overallMoveDiffY += dy;
    _overallMoveDiffX += dx;
    _lastDiffPosition = position;
  }