JB JungBae Kim

[#1809] Add a pop-up that explains the meaning of the percentage label

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Fixes #1809 ## Proposed commit message ``` When the groups are sorted by the amount of contribution, a percentage label appears; however, no other information is provided for that number,...

Unclear percentage meaning under summary-charts when groups are sorted by contribution

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**What feature(s) would you like to see in RepoSense?** When the groups are sorted by the amount of contribution, a percentage label appears; however, no other information is provided for...