function addTick(ticks, time, timestamps) {
  if (!timestamps) {
    ticks[time] = true;
  } else if (timestamps.length) {
    const {lo, hi} = _lookup(timestamps, time);
    const timestamp = timestamps[lo] >= time ? timestamps[lo] : timestamps[hi];
    ticks[timestamp] = true;
  }
}
@param {object} ticks
@param {number} time
@param {number[]} [timestamps] - if defined, snap to these timestamps