D3.js Auto font-sizing based on nodes individual radius/diameter

To offset text within a circle, rather than running along the diameter, I implement this differently:

dy shifts a text node up or down within the circle and is used to calculate the width or the chord to size the text.

The scale is then stored in a data attribute on the text element, rather than modifying the source data.

jsfiddle

function appendScaledText(parentGroup, textVal, dyShift) {
  parentGroup
    .append("text")
    .attr("dy", dyShift)
    .attr("text-anchor", "middle")
    .attr("dominant-baseline", "central")
    .attr("font-family", "sans-serif")
    .attr("fill", "white")
    .text(textVal)
    .style("font-size", "1px")
    .each(getSize)
    .style("font-size", function() {
      return d3.select(this).attr("data-scale") + "px";
    });
}

function getSize() {
  var d3text = d3.select(this);

  // in other cases could be parentElement or nextElementSibling
  var circ = d3.select(this.previousElementSibling); 
  var radius = Number(circ.attr("r"));
  var offset = Number(d3text.attr("dy"));

  // TODO: this could be bounding box instead
  var textWidth = this.getComputedTextLength();

  // TODO: could adjust based on ratio of dy to radius 
  var availWidth = chordWidth(Math.abs(offset), radius); 

  // fixed 15% 'padding', could be more dynamic/precise based on above TODOs
  availWidth = availWidth * 0.85;

  d3text.attr("data-scale", availWidth / textWidth); 
}

function chordWidth(dFromCenter, radius) {
  if (dFromCenter > radius) return Number.NaN;
  if (dFromCenter === radius) return 0;
  if (dFromCenter === 0) return radius * 2;

  // a^2 + b^2 = c^2
  var a = dFromCenter;
  var c = radius;
  var b = Math.sqrt(Math.pow(c, 2) - Math.pow(a, 2)); // 1/2 of chord length

  return b * 2;
}

You can do this by dynamically setting the text size based on the size of the container. For this, you have to add the text, get its bounding box, get the bounding box of the container element and derive the correct font size based on the current font size and those bounding boxes.

The code would look something like this.

// ...
  .append("text")
  .text("text")
  .style("font-size", "1px")
  .each(getSize)
  .style("font-size", function(d) { return d.scale + "px"; });

function getSize(d) {
  var bbox = this.getBBox(),
      cbbox = this.parentNode.getBBox(),
      scale = Math.min(cbbox.width/bbox.width, cbbox.height/bbox.height);
  d.scale = scale;
}