Need more  obvious  comparison function

User may don't know how to use comparison function.

![image](https://user-images.githubusercontent.com/31329157/117900744-763acd80-b2fc-11eb-8b5e-97211af1d509.png)

Refer comment: https://github.com/api7/contributor-graph/pull/37#issuecomment-833198871