code: function _wheelHandler(e) {
        var pos,
            ratio,
            animation,
            wheelDelta = sigma.utils.getDelta(e);
  
        if (_settings('mouseEnabled') && _settings('mouseWheelEnabled') && wheelDelta !== 0) {
          ratio = wheelDelta > 0 ?
            1 / _settings('zoomingRatio') :
            _settings('zoomingRatio');
  
          pos = _camera.cameraPosition(
            sigma.utils.getX(e) - sigma.utils.getCenter(e).x,
            sigma.utils.getY(e) - sigma.utils.getCenter(e).y,
            true
          );
  
          animation = {
            duration: _settings('mouseZoomDuration')
          };
  
          sigma.utils.zoomTo(_camera, pos.x, pos.y, ratio, animation);
  
          if (e.preventDefault)
            e.preventDefault();
          else
            e.returnValue = false;
  
          e.stopPropagation();
          return false;
        }
      }
docs: The handler listening to the 'wheel' mouse event. It will basically zoom
  in or not into the graph.
  
  @param {event} e A mouse event.