function _wheelHandler(e) {
      var pos,
          ratio,
          animation;

      if (_settings('mouseEnabled') && _settings('mouseWheelEnabled')) {
        ratio = sigma.utils.getDelta(e) > 0 ?
          1 / _settings('zoomingRatio') :
          _settings('zoomingRatio');

        pos = _camera.cameraPosition(
          sigma.utils.getX(e) - sigma.utils.getWidth(e) / 2,
          sigma.utils.getY(e) - sigma.utils.getHeight(e) / 2,
          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;
      }
    }
The handler listening to the 'wheel' mouse event. It will basically zoom
in or not into the graph.

@param {event} e A mouse event.