Simple d3.js tooltips

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

It is aimed at demonstrating a simple tooltip using d3.js and 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;
}

div.tooltip {	
    position: absolute;			
    text-align: center;			
    width: 60px;					
    height: 28px;					
    padding: 2px;				
    font: 12px sans-serif;		
    background: lightsteelblue;	
    border: 0px;		
    border-radius: 8px;			
    pointer-events: none;			
}

</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;
var formatTime = d3.time.format("%e %B");

// 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); });

// Define the div for the tooltip
var div = d3.select("body").append("div")	
    .attr("class", "tooltip")				
    .style("opacity", 0);

// 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; })]);

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

// Add the scatterplot
    svg.selectAll("dot")	
        .data(data)			
    .enter().append("circle")								
        .attr("r", 5)		
        .attr("cx", function(d) { return x(d.date); })		 
        .attr("cy", function(d) { return y(d.close); })		
        .on("mouseover", function(d) {		
            div.transition()		
                .duration(200)		
                .style("opacity", .9);		
            div	.html(formatTime(d.date) + "<br/>"  + d.close)	
                .style("left", (d3.event.pageX) + "px")		
                .style("top", (d3.event.pageY - 28) + "px");	
            })					
        .on("mouseout", function(d) {		
            div.transition()		
                .duration(500)		
                .style("opacity", 0);	
        });

// 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