code: function(evt) {
          OpenLayers.Control.PanZoom.prototype.onButtonClick.apply(this, arguments);
          if (evt.buttonElement === this.zoombarDiv) {
              var levels = evt.buttonXY.y / this.zoomStopHeight;
              if(this.forceFixedZoomLevel || !this.map.fractionalZoom) {
                  levels = Math.floor(levels);
              }    
              var zoom = (this.map.getNumZoomLevels() - 1) - levels; 
              zoom = Math.min(Math.max(zoom, 0), this.map.getNumZoomLevels() - 1);
              this.map.zoomTo(zoom);
          }
      }
docs: Method: onButtonClick
  
  Parameters:
  evt - {Event}