Non-Linear Solubility Trend
The solubility can be thought of as an equilibrium. The Van 't Hoff Equation describes the relationship between any equilibrium constant and temperature. 

$lnK_2 - lnK_1 = \frac{\Delta H }{R} (\frac{1}{T_1}-\frac{1}{T_2})$ . Thus, the graph is not linear.