With only a very few exceptions, solubility increases with temperature. The line between the yellow and magenta regions on the same figure is not exact and depends on several factors, one of which is cleanliness.
Tiny dirt can provide sites for nucleation.
Appreciating that the change from one zone to the other is an approximation, we can see three distinct regions on the graph. The lower region corresponds to permutations of temperatures and concentrations where the material is soluble. If crystals were added, they would dissolve. Although above the saturation line, there is no certainty that crystals will form. There are cases where solutions have been held for many years with no crystallization.