code: func (kde *KDE) normalizedXs(x float64) []float64 {
  	txs := make([]float64, len(kde.Sample.Xs))
  	for i, xi := range kde.Sample.Xs {
  		txs[i] = x - xi
  	}
  	return txs
  }
docs: // TODO: For KDEs of histograms, make histograms able to create a
  // weighted Sample and simply require the caller to provide a
  // good bandwidth from a StreamStats.
  // normalizedXs returns x - kde.Sample.Xs. Evaluating kernels shifted
  // by kde.Sample.Xs all at x is equivalent to evaluating one unshifted
  // kernel at x - kde.Sample.Xs.