code: function(opacity) {
          if (opacity != this.opacity) {
              this.opacity = opacity;
              for (var i=0, len=this.markers.length; i<len; i++) {
                  this.markers[i].setOpacity(this.opacity);
              }
          }
      }
docs: APIMethod: setOpacity
  Sets the opacity for all the markers.
  
  Parameters:
  opacity - {Float}