From the displacement-time graph find out the velocity of a moving body

In first instant, you will apply v = tan$\theta$ and say

$v=\tan 30°=\frac{1}{\sqrt{3}}m/s$

But it is wrong because formula v = tan$\theta$ is valid when angle is measured with time axis. 

Here, angle is taken from displacement axis. So, angle from time axis = 90^o$-$30^o=60^o

Now,            $v=\tan {60}^o=\sqrt{3}$