Answer: the name and the units of the vertical axis.
Explanation:
As you see the horizontal axis shows that the independent variable is Temperature and that it is reported in °C.
That kind of information is necessary to be shown to understand the graph. Given that the equation states that it is a solubility graph, we know that the vertical axis shows solubility, but each axis must show the name. Furthermore, we do not know the unit of measure, so when we read a number we do not know if it is g/cm^3, kg/liter, ppm, g/kg, or any other unit.
