计算中心点的算法（某些地图区域中心点计算可能会用到，只适用于x轴，y轴计算）

var points = [区域点位数组]
if(points.length > 3) {
  // 计算中间点位置
  var middle = points[Math.round(points.length / 2)]
  var first = points[0]
  var middlePoint = [Math.abs(first[0] + (middle[0] - first[0]) / 2), Math.abs(first[1] + (middle[1] - first[1]) / 2)]
} else {
  middlePoint = points[0]
}