$A$ block of mass $m$ is on an inclined plane of angle $\theta$. The coefficient of friction between the block and the plane is $\mu$ and tan $\theta >\mu.$ The block is held stationary by applying a force $P$ parallel to the plane. The direction of force pointing up the plane is taken to be positive. As $P$ is varied from $P_{1}=mg$ $\left(sin\,\theta-\mu\,cos\,\theta\right)$ to $P_{2}=mg$ $\left(sin\theta+\mu cos\theta\right)$, the frictional force $f$ versus $P$ graph will look like