Aluminium sulphate solubility as function of the pH
The lack of continuity for the curve indicates that the concentration of $\ce{Al2(SO4)3}$ was below the detection limit of the analytical technique being used to measure the concentration of $\ce{Al2(SO4)3}$ in the solution. 

Theoretically there should be a continuous curve.  

+1 for asking this as a new question.