When I was reading some miner code I saw a piece and found it was related to the nonce's value, but I was not able find out whether it is proportional or not. If not, then what is it's relation to the nonce's value?
What is relation of job target and nonce in pool mining
The job target is the level of difficulty the miner produced hash must meet to be accepted by the pool.
The nonce is a miner supplied piece of data to include in what it hashes. Every time the nonce is changed the resulting hash is different, hence the miner keeps changing the nonce till it finds a hash that meets the job difficulty target.
but I was not able find out whether it is proportional or not
They are not proportional.