feat: allow custom tooltip formatting

[lsh]

Please:

• [-] Make the pull requests (PRs) atomic (fix one issue at a time). Multiple relevant issues that must be fixed together? Make atomic commits so we can easily review each issue.
• [-] Provide a concise title as a semantic commit message (e.g. "fix: correctly handle undefined properties") so we can easily copy it to the release note.
  • Use imperative mood and present tense.
• Mention relevant issues in the description (e.g., Fixes #1 / Fixes part of #1).
• [-] Lint and test (Run yarn test).
• [-] Review your changes before sending the PR (to ensure code quality).
• For new features:
  • [-] Add new unit tests.
  • [-] Update the documentation under site/docs/ + add examples.

Tips:

• https://medium.com/@greenberg/writing-pull-requests-your-coworkers-might-enjoy-reading-9d0307e93da3 is a nice article about writing a nice PR.
• Use draft PR for work in progress PRs / when you want early feedback (https://github.blog/2019-02-14-introducing-draft-pull-requests/).

[kanitw]

Thanks!

• good to add an example spec to show that this feature work (in examples/specs)
• refer to the example in config.md

[kanitw]

Please fix minor nit before merging. :)

[kanitw]

Let's change axis format to no decimal points and we merge squash merge :)