Cool intelligent rounding function

round.js

function roundToSignificantFigures(num, sigFigs, truncationFunc) {
  if (num == 0) {
      return 0;
  }

  // We use base 5 because it makes for pretty numbers without intervals
  // quite as large as base 10.
  var d = Math.ceil(Math.log(num < 0 ? -num: num) / Math.log(5));
  var power = sigFigs - d;

  var magnitude = Math.pow(5, power);
  var shifted = truncationFunc(num*magnitude);
  return shifted / magnitude;
}

// Invoke it something like:
roundToSignificantFigures(prices.min().value(), 1, Math.floor)
// or
roundToSignificantFigures(prices.max().value(), 1, Math.ceil)

function roundToSignificantFigures(num, sigFigs, truncationFunc) {
  if (num == 0) {
      return 0;
  }
  const base = 10;

  const d = Math.ceil(Math.log(num < 0 ? -num : num) / Math.log(base));
  const power = sigFigs - d;

  const magnitude = Math.pow(base, power);
  const shifted = truncationFunc(num * magnitude);
  return shifted / magnitude;
}