* expressed as {@code y = m * x + c} for finite {@code m} and {@code c}, unless it is a vertical
 * transformation in which case {@code x} has a constant value for all {@code y}. In the
 * non-vertical case, {@code m} is the slope of the transformation (and a horizontal transformation
 * has zero slope).
 *
 * @author Pete Gillin
 * @since 20.0
 */