fix: use String#characters instead of substring for truncation in circular charts.

[btrautmann]

This PR is just being used to demonstrate that we can fix the below mentioned issue by using package:characters (which is exported via the Flutter SDK). Currently, the circular_data_label_helper uses substring when truncating text which is prone to causing issues if the input String contains, for instance, emojis like 🍕 that are comprised of multiple code points. This is explained in more detail in this comment.

Notes:

• This PR is not holistic in that it does not target all other uses of .substring, of which there seem to be 9. I just opened this to demonstrate how [syncfusion_flutter_charts] Pie Chart data labels throw Invalid argument(s): string is not well-formed UTF-16 #1611 should likely be fixed.
• I've pushed a commit to https://github.com/btrautmann/syncfusion_charts_repros that has in its pubspec.yaml the URL/ref for this changeset available for use if you want to test yourself, just comment out the hosted version and uncomment the git ref.

Before:

After:

Fixes #1611.

[PreethikaSelvam]

Hi @btrautmann,

We would like to let you know that we have logged two feature requests.

The first feature request (FR) is to reposition the data label based on the available space. In this case, the shift position will be modified based on the available size.

FR Link: https://www.syncfusion.com/feedback/42028/improve-the-data-label-positioning-based-on-the-available-place

The second FR is to restrict the data label builder size instead of hiding it based on the available size.

FR Link: https://www.syncfusion.com/feedback/50685/provide-an-option-to-trim-the-data-label-builder-instead-of-hiding-it-based-on-the

Therefore, the above two feature requests are correct and as mentioned in previous updates, they will be implemented in one of our upcoming releases.

Furthermore, we are pleased to hear that you have discovered the root cause here: https://github.com/syncfusion/flutter-widgets/pull/1661. We will check this and consider all related features to determine the feasibility of incorporating your fix into our feature request implementation while implementing this feature: https://www.syncfusion.com/feedback/50685/provide-an-option-to-trim-the-data-label-builder-instead-of-hiding-it-based-on-the.

Regards,

Preethika Selvam.