Graphically, the Engel curve is represented in the first-quadrant of the Cartesian coordinate system. Income is shown on the Y-axis and the quantity demanded for the selected good or service is shown on the X-axis. A good's Engel curve reflects its income elasticity and indicates whether the good is an inferior, normal, or luxury good. Empirical Engel curves are close to linear for some goods, and highly nonlinear for others.
For normal goods, the Engel curve has a positive gradient. That is, as income increases, the quantity demanded increases.
For inferior goods, the Engel curve has a negative gradient. That means that as the consumer has more income, they will buy less of the inferior good because they are able to purchase better goods.
So, neither Y-axis nor X-axis can be normal or inferior, because they show income and quantity demanded and the fact that the good is normal or inferior depends on the gradient of the Engel curve.