Placing labels at the center of nodes in d3.js

I am starting with d3.js, and am trying to create a row of nodes each of which contains a centered number label.

I am able to produce the desired result visually, but the way I did it is hardly optimal as it involves hard-coding the x-y coordinates for each text element. Below is the code:

var svg_w = 800;
var svg_h = 400;
var svg = d3.select("body")
    .append("svg")
    .attr("width", svg_w)
    .attr("weight", svg_h);

var dataset = [];
for (var i = 0; i < 6; i++) {
    var datum = 10 + Math.round(Math.random() * 20);
    dataset.push(datum);
}

var nodes = svg.append("g")
               .attr("class", "nodes")
               .selectAll("circle")
               .data(dataset)
               .enter()
               .append("circle")
               .attr("class", "node")
               .attr("cx", function(d, i) {
                   return (i * 70) + 50;
               })
               .attr("cy", svg_h / 2)
               .attr("r", 20);

var labels = svg.append("g")
                .attr("class", "labels")
                .selectAll("text")
                .data(dataset)
                .enter()
                .append("text")
                .attr("dx", function(d, i) {
                    return (i * 70) + 42
                })
                .attr("dy", svg_h / 2 + 5)
                .text(function(d) {
                    return d;
                });

The node class is custom CSS class I've defined separately for the circle elements, whereas classes nodes and labels are not explicitly defined and they are borrowed from this answer.

As seen, the positioning of each text label is hard-coded so that it appears at the center of the each node. Obviously, this is not the right solution.

My question is that how should I correctly associate each text label with each node circle dynamically so that if the positioning of a label changes along with that of a circle automatically. Conceptual explanation is extremely welcome with code example.

The text-anchor attribute works as expected on an svg element created by D3. However, you need to append the text and the circle into a common g element to ensure that the text and the circle are centered with one another.

To do this, you can change your nodes variable to:

var nodes = svg.append("g")
           .attr("class", "nodes")
           .selectAll("circle")
           .data(dataset)
           .enter()
           // Add one g element for each data node here.
           .append("g")
           // Position the g element like the circle element used to be.
           .attr("transform", function(d, i) {
             // Set d.x and d.y here so that other elements can use it. d is 
             // expected to be an object here.
             d.x = i * 70 + 50,
             d.y = svg_h / 2;
             return "translate(" + d.x + "," + d.y + ")"; 
           });

Note that the dataset is now a list of objects so that d.y and d.x can be used instead of just a list of strings.

Then, replace your circle and text append code with the following:

// Add a circle element to the previously added g element.
nodes.append("circle")
      .attr("class", "node")
      .attr("r", 20);

// Add a text element to the previously added g element.
nodes.append("text")
     .attr("text-anchor", "middle")
     .text(function(d) {
       return d.name;
      });

Now, instead of changing the position of the circle you change the position of the g element which moves both the circle and the text.

Here is a JSFiddle showing centered text on circles.

If you want to have your text be in a separate g element so that it always appears on top, then use the d.x and d.y values set in the first g element's creation to transform the text.

var text = svg.append("svg:g").selectAll("g")
         .data(force.nodes())
         .enter().append("svg:g");

text.append("svg:text")
    .attr("text-anchor", "middle")
    .text(function(d) { return d.name; });

text.attr("transform",  function(d) {
      return "translate(" + d.x + "," + d.y + ")"; 
    });

The best answer came from the asker himself:

> just a further observation: with only .attr("text-anchor", "middle") > for each text element, the label is at the middle horizontally but > slightly off vertically. I fixed this by adding attr("y", ".3em") > (borrowed from examples at d3.js website), which seems to work well > even for arbitrary size of node circle. However, what exactly this > additional attribute does eludes my understanding. Sure, it does > something to the y-coordinate of each text element, but why .3em in > particular? It seems almost magical to me...

Just add .attr("text-anchor", "middle") to each text element.

Example:

node.append("text")
    .attr("x", 0)
    .attr("dy", ".35em")
    .attr("text-anchor", "middle")
    .text(function(d) { return d.name; });

This page describes what's going on under the svg hood when it comes to text elements. Understanding the underlying machinery and data structures helped me get a better handle on how I had to modify my code to get it working.