Updating the data of a pack layout from JSON call and redrawing

I had a hard time understanding how the packs work. Apparently, you just send them a dataset and they return a new dataset that you can use for binding. Much simpler than I'd thought. This solution works and I think most people should be able to move on from here:

var diameter = 960,    format = d3.format(",d");var pack = d3.layout.pack()    .size([diameter - 4, diameter - 4])    .value(function(d) { return d.size; });var svg = d3.select("body").append("svg")    .attr("width", diameter)    .attr("height", diameter)  .append("g")    .attr("transform", "translate(2,2)");var node;var currentJson;var currentUrl = "data1.json";var getNewData = function() {    if(currentUrl == "data1.json") {        currentUrl = "data2.json";    }    else {        currentUrl = "data1.json";    }    d3.json(currentUrl, function(error, data) {        currentJson = data;        refresh();    });}var refresh = function() {    node = svg.selectAll(".node")                    .data(pack.nodes(currentJson));    node.enter().append("g")            .classed("node", true)            .attr("transform", function(d) { return "translate(" + d.x + "," + d.y + ")"; })        .append("circle")            .attr("r", 0)            .on("click", getNewData)            .transition()            .duration(2000)            .attr("r", function(d) { return d.r; });    node.transition()        .duration(2000)        .attr("transform", function(d) { return "translate(" + d.x + "," + d.y + ")"; });    node.select("circle")        .transition()        .duration(2000)        .attr("r", function(d) { return d.r; });}d3.select(self.frameElement).style("height", diameter + "px");getNewData();