Multi-Line Voronoi

This multi-line chart uses an invisible Voronoi tessellation to handle mouseover; the closest point to the mouse on any line is highlighted. Click the checkbox in the top-right to toggle the visibility of the Voronoi overlay.

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

.axis--y path {
  display: none;
}

.cities {
  fill: none;
  stroke: #aaa;
  stroke-linejoin: round;
  stroke-linecap: round;
  stroke-width: 1.5px;
}

.city--hover {
  stroke: #000;
}

.focus text {
  text-anchor: middle;
  text-shadow: 0 1px 0 #fff, 1px 0 0 #fff, 0 -1px 0 #fff, -1px 0 0 #fff;
}

.voronoi path {
  fill: none;
  pointer-events: all;
}

.voronoi--show path {
  stroke: red;
  stroke-opacity: 0.2;
}

#form {
  position: absolute;
  top: 20px;
  right: 30px;
}

</style>
<svg width="960" height="500"></svg>
<label id="form" for="show-voronoi">
  Show Voronoi
  <input type="checkbox" id="show-voronoi" disabled>
</label>
<script src="//d3js.org/d3.v4.min.js"></script>
<script>

var months,
    monthKeys,
    monthParse = d3.timeParse("%Y-%m");

var svg = d3.select("svg"),
    margin = {top: 20, right: 30, bottom: 30, left: 40},
    width = svg.attr("width") - margin.left - margin.right,
    height = svg.attr("height") - margin.top - margin.bottom,
    g = svg.append("g").attr("transform", "translate(" + margin.left + "," + margin.top + ")");

var x = d3.scaleTime()
    .range([0, width]);

var y = d3.scaleLinear()
    .range([height, 0]);

var voronoi = d3.voronoi()
    .x(function(d) { return x(d.date); })
    .y(function(d) { return y(d.value); })
    .extent([[-margin.left, -margin.top], [width + margin.right, height + margin.bottom]]);

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

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

x.domain(d3.extent(months));
  y.domain([0, d3.max(data, function(c) { return d3.max(c.values, function(d) { return d.value; }); })]).nice();

g.append("g")
      .attr("class", "axis axis--x")
      .attr("transform", "translate(0," + height + ")")
      .call(d3.axisBottom(x));

g.append("g")
      .attr("class", "axis axis--y")
      .call(d3.axisLeft(y).ticks(10, "%"))
    .append("text")
      .attr("x", 4)
      .attr("y", 0.5)
      .attr("dy", "0.32em")
      .style("text-anchor", "start")
      .style("fill", "#000")
      .style("font-weight", "bold")
      .text("Unemployment Rate");

g.append("g")
      .attr("class", "cities")
    .selectAll("path")
    .data(data)
    .enter().append("path")
      .attr("d", function(d) { d.line = this; return line(d.values); });

var focus = g.append("g")
      .attr("transform", "translate(-100,-100)")
      .attr("class", "focus");

focus.append("circle")
      .attr("r", 3.5);

focus.append("text")
      .attr("y", -10);

var voronoiGroup = g.append("g")
      .attr("class", "voronoi");

voronoiGroup.selectAll("path")
    .data(voronoi.polygons(d3.merge(data.map(function(d) { return d.values; }))))
    .enter().append("path")
      .attr("d", function(d) { return d ? "M" + d.join("L") + "Z" : null; })
      .on("mouseover", mouseover)
      .on("mouseout", mouseout);

d3.select("#show-voronoi")
      .property("disabled", false)
      .on("change", function() { voronoiGroup.classed("voronoi--show", this.checked); });

function mouseover(d) {
    d3.select(d.data.city.line).classed("city--hover", true);
    d.data.city.line.parentNode.appendChild(d.data.city.line);
    focus.attr("transform", "translate(" + x(d.data.date) + "," + y(d.data.value) + ")");
    focus.select("text").text(d.data.city.name);
  }

function mouseout(d) {
    d3.select(d.data.city.line).classed("city--hover", false);
    focus.attr("transform", "translate(-100,-100)");
  }
});

</script>

https://d3js.org/d3.v4.min.js