_getTimestampsForTable() {
    let timestamps = this._cache.all || [];

    if (timestamps.length) {
      return timestamps;
    }

    const data = this.getDataTimestamps();
    const label = this.getLabelTimestamps();
    if (data.length && label.length) {
      // If combining labels and data (data might not contain all labels),
      // we need to recheck uniqueness and sort
      timestamps = this.normalize(data.concat(label));
    } else {
      timestamps = data.length ? data : label;
    }
    timestamps = this._cache.all = timestamps;

    return timestamps;
  }
Returns all timestamps
@return {number[]}
@private