Multi-series Associative

Added as an example for the question asked here on Stack Overflow.

<!DOCTYPE html>
<meta charset="utf-8">
<style>

body {
  font: 10px sans-serif;
}

.axis path,
.axis line {
  fill: none;
  stroke: #000;
  shape-rendering: crispEdges;
}

.line {
  fill: none;
  stroke: steelblue;
  stroke-width: 1.5px;
}
.grid .tick {
    stroke: lightgrey;
    stroke-opacity: 0.7;
    shape-rendering: crispEdges;
}
.grid path {
  stroke-width: 0;
}

</style>
<body>
<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.17/d3.min.js"></script>
<script>

var margin = {top: 20, right: 80, bottom: 30, left: 50},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

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

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

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

var color = d3.scale.ordinal().range(["#FF0000", "#FFFF00", "#000000", "#FF00FF"]);

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

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

var line = d3.svg.line()
    .interpolate("basis")
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y(d.temperature); });

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 + ")");

function make_y_axis() {        
    return d3.svg.axis()
        .scale(y)
        .orient("left")
}

/*
var data = [
{date:"1-May-12","New York":"58.13", "San Francisco":"58.13", "Austin": "43"},
{date:"30-Apr-12","New York":"53.98" , "San Francisco":"48.13", "Austin": "53"},
{date:"27-Apr-12","New York":"67.00", "San Francisco":"38.13", "Austin": "63"},
{date:"26-Apr-12","New York":"89.70", "San Francisco":"28.13", "Austin": "73"},
{date:"25-Apr-12","New York":"99.00", "San Francisco":"18.13", "Austin": "83"}
];
*/

d3.csv("data.csv", function(error, data) {

data.forEach(function(d) {
    d.date = new Date (d.date);
});

color.domain(d3.keys(data[0]).filter(function(key) { return key !== "date"; }));

//console.log(data);

var cities = color.domain().map(function(name) {
  return {
    name: name,
    values: data.map(function(d) {
      return {date: d.date, temperature: +d[name]};
    })
  };
});

console.log(cities);

x.domain(d3.extent(data, function(d) { return d.date; }));

y.domain([
  d3.min(cities, function(c) { return d3.min(c.values, function(v) { return v.temperature; }); }),
  d3.max(cities, function(c) { return d3.max(c.values, function(v) { return v.temperature; }); })
]);

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

svg.append("g")
    .attr("class", "y axis")
    .call(yAxis)
svg.append("g")         
  .attr("class", "grid")
  .call(make_y_axis()
      .tickSize(-width, 0, 0)
      .tickFormat("")
  )

var city = svg.selectAll(".city")
    .data(cities)
  .enter().append("g")
    .attr("class", "city");

city.append("path")
    .attr("class", "line")
    .attr("d", function(d) { return line(d.values); })
    .style("stroke", function(d) { return color(d.name); });

});

</script>

https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.17/d3.min.js