Transitioning a line chart, but with data bound to the path element

Idiomatic version of an animated line chart with added animated "y-value and y-axis only" initial entry.
Instead of passing the line data into a d3.svg.line() object explicitly, the data is first bound to the path and then passed to the d3.svg.line() object implicitly via selection.attr().

<!DOCTYPE html>
<meta charset="utf-8">
<style> /* set the CSS */

body {
  font: 12px Arial;
}

path {
  stroke: steelblue;
  stroke-width: 2;
  fill: none;
}

.axis path,
.axis line {
  fill: none;
  stroke: grey;
  stroke-width: 1;
  shape-rendering: crispEdges;
}

</style>
<body>

<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js"></script>

// Set the dimensions of the canvas / graph
  var margin = {top: 30, right: 20, bottom: 30, left: 50},
    width = 600 - margin.left - margin.right,
    height = 270 - margin.top - margin.bottom;

// Parse the date / time
  var parseDate = d3.time.format("%d-%b-%y").parse;

// Set the ranges
  var x = d3.time.scale().range([0, width]);
  var y = d3.scale.linear().range([height, 0]);

// Define the axes
  var xAxis = d3.svg.axis().scale(x)
    .orient("bottom").ticks(5);

var yAxis = d3.svg.axis().scale(y)
    .orient("left").ticks(5);

// Define the line
  var valueline = d3.svg.line()
    .x(function (d) {
      return x(d.date);
    })
    .y(function (d) {
      return y(d.close);
    });

// Adds the svg canvas
  var svg = d3.select("body")
    .append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
    .append("g")
    .attr("transform",
    "translate(" + margin.left + "," + margin.top + ")");
  //EDIT ********************************************
  var line, source = ["data.csv", "data-alt.csv"];
  source.get = (function(){
    var pointer = true;
    return function(){
      return (pointer =!pointer, this[+pointer]);
    }
  })();
  // Get the data
  d3.csv(source.get(), function (error, data) {
    data.forEach(function (d) {
      d.date = parseDate(d.date);
      d.close = +d.close;
    });
    var entryData = data.map(function (d) {
      return ({date: d.date, close: 0})
    })

// Scale the range of the data
    x.domain(d3.extent(data, function (d) {
      return d.date;
    }));
    y.domain([0, d3.max(data, function (d) {
      return d.close;
    })]);

//Bind the data first THEN add the valueline path.
    //Start with all zero y values
    line = svg.selectAll("line").data([entryData]);
    line.enter().append("path")
      .attr("class", "line")
      .attr("d", valueline);
    // Transition initial data, y values only
    line.data([data]);
    line.transition().delay(500).call(trans, "entry")
      // Add the valueline path.
      .attr("d", valueline);

// Add the X Axis
    svg.append("g")
      .attr("class", "x axis")
      .attr("transform", "translate(0," + height + ")")
      .call(xAxis);

// Add the Y Axis with entry transition
    var gYaxis = svg.append("g")
      .attr("class", "y axis");
    //Null axis starting point
    y.domain([0, 0]);
    gYaxis.call(yAxis);
    //Final axis after entry transition
    y.domain([0, d3.max(data, function (d) {
      return d.close;
    })]);
    gYaxis.transition().delay(500).call(trans, "entry")
      .call(yAxis);

});

// ** Update data section (Called from the onclick)
  function updateData() {

// Get the data again
    d3.csv(source.get(), function (error, data) {
      data.forEach(function (d) {
        d.date = parseDate(d.date);
        d.close = +d.close;
      });

// Scale the range of the data again
      x.domain(d3.extent(data, function (d) {
        return d.date;
      }));
      y.domain([0, d3.max(data, function (d) {
        return d.close;
      })]);

//EDIT ********************************************
      // Bind the new data and then transition
      line = svg.selectAll(".line").data([data]);
      line.enter().append("path")
        .attr("class", "line")
      line.transition().call(trans)
        // Add the valueline path.
        .attr("d", valueline);

// Select the section we want to apply our changes to
      var svgTrans = d3.select("body").transition();

// Make the changes
      svgTrans.select(".x.axis") // change the x axis
        .call(trans)
        .call(xAxis);
      svgTrans.select(".y.axis") // change the y axis
        .call(trans)
        .call(yAxis);

});
  }
  function trans (transition, name){
    var delays = {normal:0, entry: 500};
    name = name || "normal";
    transition.duration(750).delay(delays[name]).ease("sin-in-out")
  }
</script>
</body>

https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js