Thanks for your explanations.
This concept seems more complex than I expected but I will tell what I have understood.

According to what you said and what I see in the mempool graph (I mean the picture I uploaded myself), the biggest colored band is BLUE. It means most unconfirmed transactions are related to those who have chosen between 1 and 8 sat/byte for their transactions. So, if I choose this range of fees, my transaction takes longer to be confirmed by a miner.
In addition to this, the YELLOW color seems to have a small band which means it includes a small number of unconfirmed transactions. So, if I choose anything between 80 and 140 sat/byte I have a much bigger chance to receive confirmation faster.

This is what I understand from the graph. Correct me if I'm wrong.