LinearTransformation transformation = LinearTransformation.mapping(x1, y1).withSlope(slope);
    assertDiagonalLinearTransformation(transformation, x1, y1, xDelta, xDelta * slope);
  }

  public void testMappingWithSlope_horizontal() {
    double x1 = 1.2;
    double y = 5.6;
    LinearTransformation transformation = LinearTransformation.mapping(x1, y).withSlope(0.0);
    assertHorizontalLinearTransformation(transformation, y);
  }

    LinearTransformation transformation = LinearTransformation.mapping(x1, y1).withSlope(slope);
    assertDiagonalLinearTransformation(transformation, x1, y1, xDelta, xDelta * slope);
  }

  public void testMappingWithSlope_horizontal() {
    double x1 = 1.2;
    double y = 5.6;
    LinearTransformation transformation = LinearTransformation.mapping(x1, y).withSlope(0.0);
    assertHorizontalLinearTransformation(transformation, y);
  }

  /**
   * Start building an instance which maps {@code x = x1} to {@code y = y1}. Both arguments must be
   * finite. Call either {@link LinearTransformationBuilder#and} or {@link
   * LinearTransformationBuilder#withSlope} on the returned object to finish building the instance.
   */
  public static LinearTransformationBuilder mapping(double x1, double y1) {
    checkArgument(isFinite(x1) && isFinite(y1));

    if (xSumOfSquaresOfDeltas > 0.0) {
      if (yStats.sumOfSquaresOfDeltas() > 0.0) {
        return LinearTransformation.mapping(xStats.mean(), yStats.mean())
            .withSlope(sumOfProductsOfDeltas / xSumOfSquaresOfDeltas);
      } else {
        return LinearTransformation.horizontal(yStats.mean());
      }
    } else {
      checkState(yStats.sumOfSquaresOfDeltas() > 0.0);

    if (xSumOfSquaresOfDeltas > 0.0) {
      if (yStats.sumOfSquaresOfDeltas() > 0.0) {
        return LinearTransformation.mapping(xStats.mean(), yStats.mean())
            .withSlope(sumOfProductsOfDeltas / xSumOfSquaresOfDeltas);
      } else {
        return LinearTransformation.horizontal(yStats.mean());
      }
    } else {
      checkState(yStats.sumOfSquaresOfDeltas() > 0.0);