Favorite tooltip (simple version) v5

This is a follow on from the simple line graph written with d3.js v5 and based on @mbostock's example here.

It impliments a simple tooltip which I find particularly neat. This implimentation is designed as a simple example which will be built on to create a slightly more complex example.

This graph is part of the code samples for the update to the book D3 Tips and Tricks to version 5 of d3.js.

<!DOCTYPE html>
<meta charset="utf-8">
<style> /* set the CSS */

.line {
  fill: none;
  stroke: steelblue;
  stroke-width: 2px;
}
  
</style>
<body>

<script src="https://d3js.org/d3.v5.min.js"></script>
<script>

// set the dimensions and margins of the graph
var margin = {top: 20, right: 20, bottom: 30, left: 50},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

// parse the date / time
var parseTime = d3.timeParse("%d-%b-%y");
    bisectDate = d3.bisector(function(d) { return d.date; }).left;

// set the ranges
var x = d3.scaleTime().range([0, width]);
var y = d3.scaleLinear().range([height, 0]);

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

// append the svg obgect to the body of the page
// appends a 'group' element to 'svg'
// moves the 'group' element to the top left margin
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").then(function(data) {

// format the data
  data.forEach(function(d) {
      d.date = parseTime(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")
      .data([data])
      .attr("class", "line")
      .attr("d", valueline);

// Add the X Axis
  svg.append("g")
      .attr("transform", "translate(0," + height + ")")
      .call(d3.axisBottom(x));

// Add the Y Axis
  svg.append("g")
      .call(d3.axisLeft(y));

// 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://d3js.org/d3.v5.min.js