Aluminium sulphate solubility as function of the pH

Question

earlier I asked about how to know the aluminium sulfate solubility as function of the ph (see the previous question here). I found this diagram on the page 57 of

Rubin, A. J., & Hayden, P. L. (1973). Studies on the Hydrolysis and Precipitation of Aluminum (III).

But I'm a little confused about the interpretation. Is the $\mathrm{Al_{2}(SO_{4})_{3}}$ insoluble between pH 5 and 7?. What will be the correct interpretation of this diagram?

Answer 1

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.