$\text{Resolving Component of T }$

$ T cos\theta = mg $ ............(1)

$ T sin\theta = qE = q\frac{\sigma}{\epsilon_0}$ .............(2)

$\text{Dividing equation (1)( and (2),we get}$

$ tan\theta = \frac{q\sigma}{mg\epsilon_0}$

$\Rightarrow \sigma \propto tan\theta $