docs: public void add(final DataPoints datapoints,
                    final String options) {
      // Technically, we could check the number of data points in the
      // datapoints argument in order to do something when there are none, but
      // this is potentially expensive with a SpanGroup since it requires
      // iterating through the entire SpanGroup.  We'll check this later
      // when we're trying to use the data, in order to avoid multiple passes
      // through the entire data.
      this.datapoints.add(datapoints);
      this.options.add(options);
    }