public abstract boolean isVertical();

  /** Returns whether this is a horizontal transformation. */
  public abstract boolean isHorizontal();

  /**
   * Returns the slope of the transformation, i.e. the rate of change of {@code y} with respect to
   * {@code x}. This must not be called on a vertical transformation (i.e. when {@link
   * #isVertical()} is true).
   */
  public abstract double slope();

    double x1 = 1.2;
    double y1 = 3.4;
    double slope = Double.MIN_VALUE;
    LinearTransformation transformation = LinearTransformation.mapping(x1, y1).withSlope(slope);
    assertThat(transformation.isVertical()).isFalse();
    assertThat(transformation.isHorizontal()).isFalse();
    assertThat(transformation.slope()).isWithin(ALLOWED_ERROR).of(slope);

    double x1 = 1.2;
    double y1 = 3.4;
    double slope = Double.MIN_VALUE;
    LinearTransformation transformation = LinearTransformation.mapping(x1, y1).withSlope(slope);
    assertThat(transformation.isVertical()).isFalse();
    assertThat(transformation.isHorizontal()).isFalse();
    assertThat(transformation.slope()).isWithin(ALLOWED_ERROR).of(slope);