code: func (hg *Histogram) AppendBucket(lower *types.Datum, upper *types.Datum, count, repeat int64) {
  	hg.Buckets = append(hg.Buckets, Bucket{Count: count, Repeat: repeat})
  	hg.Bounds.AppendDatum(0, lower)
  	hg.Bounds.AppendDatum(0, upper)
  }
docs: // AppendBucket appends a bucket into `hg`.