src/format/formatPrefix.js

import "precision";
import "round";
import "../math/abs";

var d3_formatPrefixes = ["y","z","a","f","p","n","µ","m","","k","M","G","T","P","E","Z","Y"].map(d3_formatPrefix);

d3.formatPrefix = function(value, precision) {
  var i = 0;
  if (value) {
    if (value < 0) value *= -1;
    if (precision) value = d3.round(value, d3_format_precision(value, precision));
    i = 1 + Math.floor(1e-12 + Math.log(value) / Math.LN10);
    i = Math.max(-24, Math.min(24, Math.floor((i <= 0 ? i + 1 : i - 1) / 3) * 3));
  }
  return d3_formatPrefixes[8 + i / 3];
};

function d3_formatPrefix(d, i) {
  var k = Math.pow(10, abs(8 - i) * 3);
  return {
    scale: i > 8 ? function(d) { return d / k; } : function(d) { return d * k; },
    symbol: d
  };
}