were extracted.
    const ids = Object.keys(idToTimestampToPoint);
    if (ids.length === 0) return undefined;

    ids.sort();
    for (let i = 0; i < dumpCount; i++) {
      let baseSize = 0;
      // Traverse |ids| in reverse (alphabetical) order so that the first id is
      // at the top of the chart.
      for (let j = ids.length - 1; j >= 0; j--) {
        const point = idToTimestampToPoint[ids[j]][i];
        point.yBase = baseSize;
        point.y += baseSize;
        baseSize = point.y;
      }
  