工具提示在D3多线图中显示NaN错误

Question

我根据自己的要求定制了D3 Multi系列折线图。我是D3图和javascript的新手。当我添加工具提示并运行代码时，它向我显示了一个带有两个NaN值的div。如果有人能在这里帮助我的话，我一直在努力解决这个问题，我将不胜感激。

    <script>

    var svg = d3.select("svg"),
        margin = {top: 20, right: 80, bottom: 30, left: 140},
        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 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); });

    var div = d3.select("body").append("div")   
        .attr("class", "tooltip")               
        .style("opacity", 0);


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

      var cities = data.columns.slice(1).map(function(id) {
        return {
          id: id,
          values: data.map(function(d) {
            return {date: d.date, temperature: d[id]};
          })
        };
      });

      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 0.02*d.temperature; }); })
      ]);

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

      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))
        .append("text")
          .attr("transform", "rotate(-90)")
          .attr("y", 6)
          .attr("dy", "0.71em")
          .attr("fill", "#000")
          .text("Time, ms");

      var city = g.selectAll(".city")
        .data(cities)
        .enter().append("g")
          .attr("class", "city");

      **city.append("path")
          .attr("class", "line")
          .attr("d", function(d) { return line(d.values); })
          .style("stroke", function(d) { return z(d.id); })
          .on("mouseover",  function(d) {       
                div.transition()        
                    .duration(200)      
                    .style("opacity", .9);      
                div .html(x(d.date)+"<br/>"  + y(d.temperature))    
                    .style("left", (d3.event.pageX) + "px")     
                    .style("top", (d3.event.pageY - 28) + "px");    
                })                  
            .on("mouseout", function(d) {       
                div.transition()        
                    .duration(500)      
                    .style("opacity", 0);   
            });**


      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",1)
          .attr("dy", "0.38em")
          .style("width","50px")
          .style("font", "bold 10px sans-serif")
          .text(function(d) { return d.id; });
    });

    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>