That moment when you realize that, despite its numerous warts, it's possible to love JavaScript. This straghtforward construct just saved me 80 lines of cut and pasted code.

bar_class.js

var default_value = parseInt(details.default_value);
function bar_class(d) {
  if (default_value >= d.x && default_value < d.x + d.dx) {
    return "default";
  } else if (median >= d.x && median < d.x + d.dx) {
    return "median";
  }
  return "standard";
}