Favorite tooltip (simple version)

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

It impliments a simple tooltip which can also be found here and here. This implimentation is designed as a simple example which will be built on to create a slightly more complex example. 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;
}

</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;
    bisectDate = d3.bisector(function(d) { return d.date; }).left;

// 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 + ")");

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

var focus = svg.append("g") 
    .style("display", "none");

// Get the data
d3.csv("atad.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.
    lineSvg.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);

// append the circle at the intersection 
    focus.append("circle")
        .attr("class", "y")
        .style("fill", "none")
        .style("stroke", "blue")
        .attr("r", 4);
    
    // append the rectangle to capture mouse
    svg.append("rect")
        .attr("width", width)
        .attr("height", height)
        .style("fill", "none")
        .style("pointer-events", "all")
        .on("mouseover", function() { focus.style("display", null); })
        .on("mouseout", function() { focus.style("display", "none"); })
        .on("mousemove", mousemove);

function mousemove() {
		var x0 = x.invert(d3.mouse(this)[0]),
		    i = bisectDate(data, x0, 1),
		    d0 = data[i - 1],
		    d1 = data[i],
		    d = x0 - d0.date > d1.date - x0 ? d1 : d0;

focus.select("circle.y")
		    .attr("transform",
		          "translate(" + x(d.date) + "," +
		                         y(d.close) + ")");
	}

});

</script>
</body>

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