js/leaflet.markercluster.js
function (marker) {
    var addedCount,
        addedLatLng = marker._wLatLng || marker._latlng;

    if (marker instanceof L.MarkerCluster) {
      this._bounds.extend(marker._bounds);
      addedCount = marker._childCount;
    } else {
      this._bounds.extend(addedLatLng);
      addedCount = 1;
    }

    if (!this._cLatLng) {
      // when clustering, take position of the first point as the cluster center
      this._cLatLng = marker._cLatLng || addedLatLng;
    }

    // when showing clusters, take weighted average of all points as cluster center
    var totalCount = this._childCount + addedCount;

    //Calculate weighted latlng for display
    if (!this._wLatLng) {
      this._latlng = this._wLatLng = new L.LatLng(addedLatLng.lat, addedLatLng.lng);
    } else {
      this._wLatLng.lat = (addedLatLng.lat * addedCount + this._wLatLng.lat * this._childCount) / totalCount;
      this._wLatLng.lng = (addedLatLng.lng * addedCount + this._wLatLng.lng * this._childCount) / totalCount;
    }
  }
function (marker) {
    var addedCount,
        addedLatLng = marker._wLatLng || marker._latlng;

    if (marker instanceof L.MarkerCluster) {
      this._bounds.extend(marker._bounds);
      addedCount = marker._childCount;
    } else {
      this._bounds.extend(addedLatLng);
      addedCount = 1;
    }

    if (!this._cLatLng) {
      // when clustering, take position of the first point as the cluster center
      this._cLatLng = marker._cLatLng || addedLatLng;
    }

    // when showing clusters, take weighted average of all points as cluster center
    var totalCount = this._childCount + addedCount;

    //Calculate weighted latlng for display
    if (!this._wLatLng) {
      this._latlng = this._wLatLng = new L.LatLng(addedLatLng.lat, addedLatLng.lng);
    } else {
      this._wLatLng.lat = (addedLatLng.lat * addedCount + this._wLatLng.lat * this._childCount) / totalCount;
      this._wLatLng.lng = (addedLatLng.lng * addedCount + this._wLatLng.lng * this._childCount) / totalCount;
    }
  }
Expand our bounds and tell our parent to