Error: <path> attribute d: Expected number, “MNaN,

I'm tring to create a multi-line chart using D3.js v4. I'm using this example: https://bl.ocks.org/mbostock/3884955.

Sample csv Data:

storageSystem,poolId,availableVolumeCapacity,date
system01,0,18031398,20170413
system01,1,15626268,20170413
system01,2,61256286,20170413
system01,3,119514990,20170413
system02,0,15046668,20170413
system02,1,12486558,20170413
system02,2,12303396,20170413
system03,0,35171335,20170413
system03,1,17263722,20170413
system01,0,18031387,20170414
system01,1,15626257,20170414
system01,2,61256275,20170414
system01,3,119514989,20170414
system02,0,15046657,20170414
system02,1,12486547,20170414
system02,2,12303385,20170414
system03,0,35171324,20170414
system03,1,17263711,20170414

Data Object:

0: Object
  color: "#8c564b"
  key: "system03"
  values: Array(2)
    0: Object
      key: "0"
      values: Array(23)
        0: Object
          availableVolumeCapacity: 35171335
          date: Thu Apr 13 2017 00:00:00 GMT+0000 (Coordinated Universal Time)
          poolId: "0"
          storageSystem: "system03"
    1: Object
      key: "1"
      values: Array(23)
        0: Object
          availableVolumeCapacity: 17263722
          date: Thu Apr 13 2017 00:00:00 GMT+0000 (Coordinated Universal Time)
          poolId:"1"
          storageSystem: "system03"

D3.js Code:

var svg = d3.select("svg")
    m = {top: 20, right: 20, bottom: 50, left: 20},
    w = svg.attr("width") - m.left - m.right,
    h = svg.attr("height") - m.top - m.bottom,
    g = svg.append("g").attr("transform", "translate(" + m.left + "," + m.top + ")");

var parseTime = d3.timeParse("%Y%m%d");

var x = d3.scaleTime().range([0, w]),
    y = d3.scaleLinear().range([h, 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.availableVolumeCapacity); });

d3.csv("ssystem.csv", function(error, data) {
    if (error) throw error;
    data.forEach(function(d) {
            d.date = parseTime(d.date);
            d.availableVolumeCapacity = +d.availableVolumeCapacity;
});

x.domain(d3.extent(data, function(d) { return d.date; }));
y.domain([0, d3.max(data, function(d) { return d.availableVolumeCapacity; })]);

var dataNest = d3.nest()
    .key(function(d) {return d.storageSystem; })
    .key(function(d) {return d.poolId; })
    .entries(data);

console.log(dataNest)

legendSpace = w/dataNest.length;

dataNest.forEach(function(d,i) {
    svg.append("path")
        .attr("class", "line")
        .style("stroke", function() {
            return d.color = z(d.key); })
        .attr("id", 'tag'+d.key.replace(/\s+/g, ''))
        .attr("d", line(d.values));
    svg.append("text")
        .attr("x", (legendSpace/2)+i*legendSpace)
        .attr("y", h + (m.bottom/2)+ 5)
        .attr("class", "legend")
        .style("fill", function() {
            return d.color = z(d.key); })
        .on("click", function(){
            // Determine if current line is visible
            var active = d.active ? false : true,
            newOpacity = active ? 0 : 1;
            // Hide or show the elements based on the ID
            d3.select("#tag"+d.key.replace(/\s+/g, ''))
                .transition().duration(100)
                .style("opacity", newOpacity);
            // Update whether or not the elements are active
            d.active = active;
            })
        .text(d.key);
});

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

svg.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("Capacity (MB)");
});

I'm seeing the following error 4 times from the console:

Error: <path> attribute d: Expected number, "MNaN,NaNLNaN,NaN".
(anonymous) @ d3.v4.min.js:205
ul @ d3.v4.min.js:3768
al @ d3.v4.min.js:3775
(anonymous) @ multi-line.js:51
(anonymous) @ multi-line.js:45
(anonymous) @ d3.v4.min.js:5857
call @ d3.v4.min.js:3622
e @ d3.v4.min.js:5840

Any help is much appreciated.

回答1:

The root of your problem is that the value d in your anonymous function passed to dataNest.forEach still contains one level of nesting but you are using it as if it doesn't have any nesting. Your code fails because it is looking for date and availableVolumeCapacity properties on an object that only has key and values properties.

There are ultimately two approaches to fix this:

Use a single key function which combines both the storage system and pool ID. This reduces the nesting to only one level:
```
var dataNest = d3.nest()
    .key(function(d) { return d.storageSystem + " " + d.poolId; })
    .entries(data);
```
Use two nested calls to forEach:
```
dataNest.forEach(function(d2,i2) {
    d2.forEach(function (d,i) {
        svg.append("path")
        // .... rest of inner function omitted
    });
});
```
You will need to go through all uses of d (in particular d.key) and i within the inner function (whose body I omitted for brevity) to see whether you need to incorporate d2 and i2. For example, perhaps you want to use d2.key + " " + d.key instead of d.key.

回答2:

When defining your path:

svg.append("path")
    .attr("class", "line")
    .style("stroke", function() {
        return d.color = z(d.key); })
    .attr("id", 'tag'+d.key.replace(/\s+/g, ''))
    .attr("d", line(d.values)); // d.values here is not defined

you are using what was given to you in the tutorial, but each item in dataNest does not have a values property.