code: function _update() {
    var nodesData = _treeLayout.nodes(_treeData),
        linksData = _getLinks(nodesData);
  
    _normalize(nodesData);
  
    var nodes = _g.selectAll('g.node')
      .data(nodesData, _getNodeId);
    _updateNodes(nodes, _treeData);
  
    var links = _g.selectAll('path.link')
        .data(linksData, _getLinkId);
    _updateLinks(links, _treeData);
  
    nodesData.forEach(_stashOldPositions);
  }
docs: Updates position of nodes and links based on changes in _treeData.