X-Value Mouseover

A line chart with mouseover so that you can read the y-value based on the closest x-value. The x-value is found using d3.mouse and scale.invert. The y-value is then found by bisecting the data.

<!DOCTYPE html>
<meta charset="utf-8">
<style>

body {
  font: 10px sans-serif;
}

.axis path,
.axis line {
  fill: none;
  stroke: #000;
  shape-rendering: crispEdges;
}

.x.axis path {
  display: none;
}

.line {
  fill: none;
  stroke: steelblue;
  stroke-width: 1.5px;
}

.overlay {
  fill: none;
  pointer-events: all;
}

.focus circle {
  fill: none;
  stroke: steelblue;
}

</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

var margin = {top: 20, right: 50, bottom: 30, left: 50},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var parseDate = d3.time.format("%d-%b-%y").parse,
    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");

var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left");

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

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 + ")");

d3.tsv("data.tsv", function(error, data) {
  if (error) throw error;

data.forEach(function(d) {
    d.date = parseDate(d.date);
    d.close = +d.close;
  });

data.sort(function(a, b) {
    return a.date - b.date;
  });

x.domain([data[0].date, data[data.length - 1].date]);
  y.domain(d3.extent(data, function(d) { return d.close; }));

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

svg.append("g")
      .attr("class", "y axis")
      .call(yAxis)
    .append("text")
      .attr("transform", "rotate(-90)")
      .attr("y", 6)
      .attr("dy", ".71em")
      .style("text-anchor", "end")
      .text("Price ($)");

svg.append("path")
      .datum(data)
      .attr("class", "line")
      .attr("d", line);

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

focus.append("circle")
      .attr("r", 4.5);

focus.append("text")
      .attr("x", 9)
      .attr("dy", ".35em");

svg.append("rect")
      .attr("class", "overlay")
      .attr("width", width)
      .attr("height", height)
      .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.attr("transform", "translate(" + x(d.date) + "," + y(d.close) + ")");
    focus.select("text").text(formatCurrency(d.close));
  }
});

</script>

https://d3js.org/d3.v3.min.js