public void testLeastSquaresFit() {
    assertThrows(IllegalStateException.class, () -> emptyAccumulator.leastSquaresFit());
    assertThrows(
        IllegalStateException.class,
        () -> emptyAccumulatorByAddAllEmptyPairedStats.leastSquaresFit());
    assertThrows(IllegalStateException.class, () -> oneValueAccumulator.leastSquaresFit());
    assertThrows(
        IllegalStateException.class,

   * @throws IllegalStateException if the dataset is empty or contains a single pair of values, or
   *     both the {@code x} and {@code y} dataset have zero population variance
   */
  public final LinearTransformation leastSquaresFit() {
    checkState(count() > 1);
    if (isNaN(sumOfProductsOfDeltas)) {
      return LinearTransformation.forNaN();
    }
    double xSumOfSquaresOfDeltas = xStats.sumOfSquaresOfDeltas();

   * @throws IllegalStateException if the dataset is empty or contains a single pair of values, or
   *     both the {@code x} and {@code y} dataset must have zero population variance
   */
  public LinearTransformation leastSquaresFit() {
    checkState(count() > 1);
    if (isNaN(sumOfProductsOfDeltas)) {
      return LinearTransformation.forNaN();
    }
    double xSumOfSquaresOfDeltas = xStats.sumOfSquaresOfDeltas();