AreaChart: Respect baseValue of AreaChart, but allow override from Area

[nikolasrieble]

Fix a bug, introduced in #3013, which neglects to respect the AreaChart baseValue and thereby breaks existing charts.

Related Issue

#3051

How Has This Been Tested?

I tested the change locally by running the demo project, verifying that the baseValue of the AreaChart is effective by itself, and that overriding it per Area component is possible.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[nikolasrieble]

TODO:

• Add test to document the target behaviour and prevent future deviation / breaking.
• Document feature on the website.

[ckifer]

Code change looks good to me! Easy fix

@nikolasrieble Question would be do you want me to merge this now or wait for the TODO items?

[nikolasrieble]

I guess as this is a fix restoring a previously existing functionality, this is good to go.
Tests and docs are missing across the bench either way.