Failure

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

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

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

</style>
<svg width="960" height="500"></svg>
<script src="//d3js.org/d3.v4.min.js"></script>
<script>

var cities;

var svg = d3.select("svg"),
    margin = {top: 20, right: 80, bottom: 30, left: 50},
    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 parseTime = d3.timeParse("%Y%m%d");

var xGroup =  g.append("g");   
var yGroup = g.append("g");

var x = d3.scaleTime().range([0, width]),
    y = d3.scaleLinear().range([height, 0]),
    z = d3.scaleOrdinal(d3.schemeCategory10);

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

g.append("clipPath")
    .attr("id", "clip")
  .append("rect")
    .attr("width", width)
    .attr("height", height);

var zoom = d3.zoom()
    .scaleExtent([1, Infinity])
    .translateExtent([[0, 0], [width, height]])
    .on("zoom", zoomed);

var zoomRect = svg.append("rect")
    .attr("width", width)
    .attr("height", height)
    .attr("fill", "none")
    .attr("pointer-events", "all")
    .call(zoom);

d3.selection.prototype.moveToFront = function() {  
      return this.each(function(){
        this.parentNode.parentNode.appendChild(this.parentNode);
      });
    };

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

cities = data.columns.slice(1).map(function(id) {
    return {
      id: id,
      values: data.map(function(d) {
        return {date: d.date, temperature: d[id]};
      })
    };
  });
  var xExtent = d3.extent(data, function(d) {
    return d.date;
  });
  zoom.translateExtent([[x(xExtent[0]), -Infinity], [x(xExtent[1]), Infinity]]);
  x.domain(d3.extent(data, function(d) { return d.date; }));

y.domain([
    d3.min(cities, function(c) { return d3.min(c.values, function(d) { return d.temperature; }); }),
    d3.max(cities, function(c) { return d3.max(c.values, function(d) { return d.temperature; }); })
  ]);

z.domain(cities.map(function(c) { return c.id; }));

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

yGroup.attr("class", "axis axis--y")
      .call(d3.axisLeft(y))
    .append("text")
      .attr("transform", "rotate(-90)")
      .attr("y", 6)
      .attr("dy", "0.71em")
      .attr("fill", "#000")
      .text("Temperature, ºF");

yGroup.call(d3.axisLeft(y)).select(".domain").remove();

var city = g.selectAll(".city")
    .data(cities)
    .attr("clip-path", "url(#clip)")
    .enter().append("g")
      .attr("class", "city");

city.append("path")
      .attr("class", "line")
      .attr("d", function(d) { return line(d.values); })
      .attr("id", function(d) {
        return d.id.substring(0, 3).toUpperCase();
      })
      .style("stroke", function(d) { return z(d.id); });

city.append("text")
      .datum(function(d) { return {id: d.id, value: d.values[d.values.length - 1]}; })
      .attr("transform", function(d) { return "translate(" + x(d.value.date) + "," + y(d.value.temperature) + ")"; })
      .attr("x", 3)
      .attr("dy", "0.35em")
      .style("font", "10px sans-serif")
      .text(function(d) { return d.id; })
      .on("click", function(d) {
        var newOpacity;
        var thisCity = d.id.substring(0, 3).toUpperCase();

if (d3.select("#" + thisCity).style("opacity") == 1) {
          newOpacity = 0;
        } else {
          newOpacity = 1;
        }
        d3.select("#" + thisCity).style("opacity", newOpacity);
      })
      .on("mouseover", function(d){
        var thisCity = d.id.substring(0, 3).toUpperCase();
        var active = d3.select("#" + thisCity).style("opacity") == 1;
        d3.select("#" + thisCity).moveToFront()

if (!active) {
          d3.select("#" + thisCity).style("opacity", 0.75);
        }
        city.selectAll("path").filter(function(d) {
          return d.id.substring(0, 3).toUpperCase() != thisCity && d3.select("#" + d.id.substring(0, 3).toUpperCase()).style("opacity") != 0;
        }).style("stroke", "lightgrey");
      })
      .on("mouseout", function(d) {
        var thisCity = d.id.substring(0, 3).toUpperCase();

city.selectAll("path").style("stroke", function(d) {
          return z(d.id);
        })
        if (d3.select("#" + thisCity).style("opacity") == 0.75) {
          d3.select("#" + thisCity).style("opacity", 0);
        }
      });

zoomRect.call(zoom.transform, d3.zoomIdentity);
});

function zoomed() {
  var xz = d3.event.transform.rescaleX(x);
  x.domain(xz.domain());
  xGroup.call(d3.axisBottom(x).scale(xz));
  line.x(function(d) {
    return x(d.date);
  })
  g.selectAll(".city")
    .selectAll("path")
    .attr("d", line);
}

function type(d, _, columns) {
  d.date = parseTime(d.date);
  for (var i = 1, n = columns.length, c; i < n; ++i) d[c = columns[i]] = +d[c];
  return d;
}

</script>

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