code: function(node, canvas, callback) {
       var level = this.getRightLevelToShow(node, canvas), 
           fx = this.viz.labels,
           opt = $.merge({
             execShow:true,
             execHide:true,
             onHide: $.empty,
             onShow: $.empty
           }, callback || {});
       node.eachLevel(0, this.config.levelsToShow, function(n) {
           var d = n._depth - node._depth;
           if(d > level) {
               opt.onHide(n);
               if(opt.execHide) {
                 n.drawn = false; 
                 n.exist = false;
                 fx.hideLabel(n, false);
               }
           } else {
               opt.onShow(n);
               if(opt.execShow) {
                 n.exist = true;
               }
           }
       });
       node.drawn= true;
    }
docs: /*
  Hides levels of the tree until it properly fits in canvas.