ticks.js

var e10 = Math.sqrt(50),
    e5 = Math.sqrt(10),
    e2 = Math.sqrt(2);

export default function(start, stop, count) {
  var reverse = stop < start,
      i = -1,
      n,
      ticks,
      step;

  if (reverse) n = start, start = stop, stop = n;

  if ((step = tickIncrement(start, stop, count)) === 0 || !isFinite(step)) return [];

  if (step > 0) {
    start = Math.ceil(start / step);
    stop = Math.floor(stop / step);
    ticks = new Array(n = Math.ceil(stop - start + 1));
    while (++i < n) ticks[i] = (start + i) * step;
  } else {
    start = Math.floor(start * step);
    stop = Math.ceil(stop * step);
    ticks = new Array(n = Math.ceil(start - stop + 1));
    while (++i < n) ticks[i] = (start - i) / step;
  }

  if (reverse) ticks.reverse();

  return ticks;
}

export function tickIncrement(start, stop, count) {
  var step = (stop - start) / Math.max(0, count),
      power = Math.floor(Math.log(step) / Math.LN10),
      error = step / Math.pow(10, power);
  return power >= 0
      ? (error >= e10 ? 10 : error >= e5 ? 5 : error >= e2 ? 2 : 1) * Math.pow(10, power)
      : -Math.pow(10, -power) / (error >= e10 ? 10 : error >= e5 ? 5 : error >= e2 ? 2 : 1);
}

export function tickStep(start, stop, count) {
  var step0 = Math.abs(stop - start) / Math.max(0, count),
      step1 = Math.pow(10, Math.floor(Math.log(step0) / Math.LN10)),
      error = step0 / step1;
  if (error >= e10) step1 *= 10;
  else if (error >= e5) step1 *= 5;
  else if (error >= e2) step1 *= 2;
  return stop < start ? -step1 : step1;
}