simple D3 line chart

copied from: https://leanpub.com/D3-Tips-and-Tricks/read#leanpub-auto-starting-with-a-basic-graph

by gdicerbo

CSS

body {
    font: 12px Arial;
}
path {
    stroke: #588ebb;
    stroke-width: 2;
    fill: none;
}

.axis path, .axis line {
    fill: none;
    stroke: grey;
    stroke-width: 1;
    shape-rendering: crispEdges;
}
.area {
    fill: #588ebb;
    stroke-width: 0;
            opacity: .2;
}
.area1 {
    fill: #BB588E;
    stroke-width: 0;
    opacity: .2;
}
.area2 {
    fill:#FB7E2B;
    stroke-width: 0;
        opacity: .2;
}
.area3 {
    fill: #FB7E2B;
    stroke-width: 0;
            opacity: .2;
}

JavaScript

var margin = {
    top: 30,
    right: 20,
    bottom: 30,
    left: 80
};
var width = 600 - margin.left - margin.right;
var height = 270 - margin.top - margin.bottom;

var parseDate = d3.time.format("%Y").parse;
var bisectDate = d3.bisector(function(d) { return d.date; }).left,
    formatValue = d3.format(",2f"),
    formatCurrency = function(d) { return "$" + formatValue(d); };

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").ticks(10)
.tickFormat(d3.time.format("%Y"));

var yAxis = d3.svg.axis().scale(y)
    .orient("left").ticks(10).tickFormat(d3.format("$s"));

var area = d3.svg.area()
    .x(function(d) { return x(d.date); })
    .y0(height)
    .y1(function(d) { return y(d.close); });
 
 var area1 = d3.svg.area()
    .x(function(d) { return x(d.date); })
    .y0(height)
    .y1(function(d) { return y(d.open); });
 
  var area2 = d3.svg.area()
    .x(function(d) { return x(d.date); })
    .y0(height)
    .y1(function(d) { return y(d.ds); });

  var area3 = d3.svg.area()
    .x(function(d) { return x(d.date); })
    .y0(height)
    .y1(function(d) { return y(d.dr); });

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

var valueline2 = d3.svg.line()
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y(d.open); });

var valueline3 = d3.svg.line()
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y(d.ds); });

var valueline4 = d3.svg.line()
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y(d.dr); });


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 + ")");

var lineSvg = svg.append("g");

var focus = svg.append("g") 
   ...