[syncfusion_flutter_chart] Implement dynamic color for tracking ball line

[KrohnMi]

• Added logic to update the tracking ball line color dynamically based on axes value
• Updated rendering engine to reflect color changes in real-time.

[AswiniDileep]

Hi KrohnMi,

Thank you for your contribution and for submitting a pull request regarding dynamic color support for the trackball line in our Flutter Chart. We sincerely appreciate the time and effort you invested, as well as the insights you’ve shared with us.

At this time, Syncfusion does not accept external source‑code contributions for certain components, including our Flutter Chart library. This approach helps us ensure that all changes align with our internal architecture, product roadmap, and quality standards. Processing external PRs requires a comprehensive internal review and planning cycle, and for these components, this process is handled strictly within our team to maintain product consistency.

We reviewed your pull request and would like to let you know that the trackball line color can already be customized dynamically at the sample level. We have validated this internally and have also attached a sample code snippet for your reference.

Currently, we are monitoring the demand for this feature at the source-level. Until then, we will be closing this PR.

Thank you again for your understanding, your contribution, and your continued support. We look forward to hearing more of your valuable feedback in the future.

Regards,
Aswini S.

Sample:
gh_2515.zip

Demo:
dynamic_color_update_trackball_line.zip