The Y-axis is also known as growth axis because

Y-axis It is also called as growth-axis angle, measured as the posterior inferior angle on S–Gn–FHP. The Y-axis indicates the degree of downward and forward position of the chin in relation to the upper face. A large Y-axis is indicative of the clockwise rotation of the mandible. Y-axis does not give an indication of the size of the lower jaw but the direction in which it is growing. Mean 59.4°, range 53°–66°.