fix: Always start y-axis for area chart at zero

[avinashbot]

Description

Changes the default baseline calculation for when a baseline isn't manually provided, so should be safe. This also gets closer to how the other (legacy) charts behave, so the different charts types are more consistent when placed next to each other.

Related links, issue #, if available: AWSUI-62336

How has this been tested?

Updated unit tests. Some permutation tests should change as well.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.71%. Comparing base (8960f56) to head (d104736).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5125   +/-   ##
=======================================
  Coverage   97.71%   97.71%           
=======================================
  Files         991      991           
  Lines       31834    31836    +2     
  Branches    11757    11759    +2     
=======================================
+ Hits        31105    31108    +3     
+ Misses        722      682   -40     
- Partials        7       46   +39     

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