code: function getPointsBox(points) {
    if (Util.isEmpty(points)) {
      return null;
    }
  
    var minX = points[0].x;
    var maxX = points[0].x;
    var minY = points[0].y;
    var maxY = points[0].y;
    Util.each(points, function (point) {
      minX = minX > point.x ? point.x : minX;
      maxX = maxX < point.x ? point.x : maxX;
      minY = minY > point.y ? point.y : minY;
      maxY = maxY < point.y ? point.y : maxY;
    });
    return {
      minX: minX,
      maxX: maxX,
      minY: minY,
      maxY: maxY,
      centerX: (minX + maxX) / 2,
      centerY: (minY + maxY) / 2
    };
  }
docs: 获取图形的包围盒