code: @Override
    public void downsample(final long interval, final Aggregator downsampler) {
      if (downsampler == Aggregators.NONE) {
        throw new IllegalArgumentException("cannot use the NONE "
            + "aggregator for downsampling");
      }
      downsample(interval, downsampler, FillPolicy.NONE);
    }
docs: Sets an optional downsampling function with interpolation on this query.
  @param interval The interval, in milliseconds to rollup data points
  @param downsampler An aggregation function to use when rolling up data points
  @throws NullPointerException if the aggregation function is null
  @throws IllegalArgumentException if the interval is not greater than 0