Consider adding golden tests

When developing new features it's important to avoid regressions.

Since this package is very UI-specific, golden tests would be useful to ensure that nothing breaks.

While I'm working on rounded section radius for the pie chart, I've noticed some issues with my implementation, that would be easier to spot early on with golden tests that provide a reference image.

There is a package called [alchemist](https://pub.dev/packages/alchemist) that can help implement golden tests.

No file or test entry point is named. Start by reviewing the package's Flutter widget-test setup and the Alchemist documentation, then define representative chart reference images and verify that the golden suite catches the rounded-section regression.

Written by the indexing model from the issue text.