d3.js - parseDate = d3.time.format("%Y") not working

Question

My chart is working completely as expected, except that my parseDate function doesn't want to give me correct dates on the x-axis. I'm sure this is something simple.

Currently I'm adding d3.v2.min.js, without any additional helper libraries - do I need something else to get d3.time.format() working?

Without parsing the date my data returns an x-axis with:

.960   .965   .970   .975

rather than

1960   1965   1970   1975

The JSON is structured like this:

[{"year":1959,"average":315.97},
{"year":1960,"average":316.91},
{"year":1961,"average":317.64},
...etc
{"year":2011,"average":391.57}]

The code with the // commented sections being the issue:

<script>
var margin = {top: 20, right: 20, bottom: 30, left: 50},
    width = 920 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

// parseDate below not working

var parseDate = d3.time.format("%Y").parse;

var x = d3.time.scale()
    .range([0, width]);

var y = d3.scale.linear()
    .range([height, 0]);

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom");

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

// data taking format  
d3.json("{{ asset('js/data_co2.json') }}", function(data) {

// parseDate() not working below

Gives error: Uncaught TypeError: Object 1959 has no method 'substring'

  data.forEach(function(d) {
    d.year = parseDate(d.year);
    d.average = +d.average;
  });

  x.domain(d3.extent(data, function(d) { return d.year; }));
  y.domain([260, 420]);

var area = d3.svg.area()
    .x(function(d) { return x(d.year); })
    .y0(height)
    .y1(function(d) { return y(d.average); });

var svg = d3.select("#co2").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 + ")");

svg.append("path")
  .datum(data)
    .attr("class", "area")
    .attr("d", area);

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

svg.append("g")
    .attr("class", "y axis")
  .call(yAxis)
    .append("text")
    .attr("transform", "rotate(-90)")
    .attr("y", 6)
    .attr("dy", ".71em")
  .style("text-anchor", "end")
  .text("CO2 Levels (ppm)");

});

</script>

Answer 1

parseDate, the function "prepared" by d3, expects to be passed a string for parsing. Your years are (appropriately) numbers, so you would need to convert them to strings:

parseDate(String(d.year));

or

parseDate(d.year.toString());

However, keep in mind that the same result – a Date object – could be achieved natively, like this:

var date = new Date(d.year, 0);// The required 0, is for January