Colour gradient on a graph line

This is a follow on from the simple d3.js graph used as an example in the book D3 Tips and Tricks.

It is aimed at demonstrating the application of a colour gradient to the line of a line graph using d3.js. It should be taken in context with the text of the book which can be downloaded for free from Leanpub.

<!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;
}

.line {
    fill: none;
    stroke: url(#line-gradient);
    stroke-width: 2px;
}

</style>
<body>

<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.17/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 + ")");

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

// 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; })]);

svg.append("linearGradient")				
        .attr("id", "line-gradient")			
        .attr("gradientUnits", "userSpaceOnUse")	
        .attr("x1", 0).attr("y1", y(0))			
        .attr("x2", 0).attr("y2", y(1000))		
    .selectAll("stop")						
        .data([								
            {offset: "0%", color: "red"},		
            {offset: "40%", color: "red"},	
            {offset: "40%", color: "black"},		
            {offset: "62%", color: "black"},		
            {offset: "62%", color: "lawngreen"},	
            {offset: "100%", color: "lawngreen"}	
        ])					
    .enter().append("stop")			
        .attr("offset", function(d) { return d.offset; })	
        .attr("stop-color", function(d) { return d.color; });

// Add the valueline path.
    svg.append("path")
        .attr("class", "line")
        .attr("d", valueline(data));

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

// Add the Y Axis
    svg.append("g")
        .attr("class", "y axis")
        .call(yAxis);

});

</script>
</body>

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