feat: Enhanced line chart curve function

[huanghui1998hhh]

This PR enhances the curve API:

1. Keeps the existing curve algorithm.
2. Add a new curve algorithm copied from ECharts.
3. Allows users to provide their own curve drawing logic in whatever way they prefer.
4. Fully preserved the existing implicit animation when toggling between curved and straight lines.
5. The old API has not been removed. It is marked as deprecated and remains backward-compatible.

Before (old params)	After (new param)
isCurved: false	curve: LineChartCurve.noCurve
isCurved: true	curve: const LineChartCubicTensionCurve()
isCurved: true, curveSmoothness: S, preventCurveOverShooting: P, preventCurveOvershootingThreshold: T	curve: const LineChartCubicTensionCurve(smoothness: S, preventCurveOverShooting: P, preventCurveOvershootingThreshold: T)

I’m not entirely sure whether these decisions align with your original vision, please give me some advice. I'll complete the follow-up work.
@imaNNeo

[codecov]

Codecov Report

❌ Patch coverage is 99.31973% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.97%. Comparing base (af78a41) to head (3c31543).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/src/chart/line_chart/line_chart_curve.dart	99.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2000      +/-   ##
==========================================
+ Coverage   92.55%   92.97%   +0.41%     
==========================================
  Files          50       51       +1     
  Lines        3760     3884     +124     
==========================================
+ Hits         3480     3611     +131     
+ Misses        280      273       -7     

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[imaNNeo]

It looks good!
Can you please write the needed unit-tests and update the documentation? both in our dart code and markdown files.

[huanghui1998hhh]

@imaNNeo Done! Please review.

[afijal]

This works a lot better than og algo, any chance of merging it?

[huanghui1998hhh]

@imaNNeo Hi~ Any work on it?

[imaNNeo]

There's a typo in _resovleCurve

[imaNNeo]

Why do we need to keep a mutable static field inside our const LineChartCubicTensionCurve()?
Is it needed? Can we remove that?

[imaNNeo]

The same for here

[imaNNeo]

Can we remove these static functions? Because it's not scalable, for every new type we should add a new function here.
(We can keep the noCurve, but let's not add others).

I see that we use them only for testing. So we can use the constructor LineChartCubicTensionCurve() instead.

[imaNNeo]

The same for here

[imaNNeo]

I think with EquatableMixin is redundant here (we can remove it)

[imaNNeo]

I don't understand why we need to have this Generic type here? <T extends LineChartCurve<T>>

We can have the lerp function like this:

LineChartCurve lerp(covariant LineChartCurve other, double t);

And it already works well! (without the need to have a generic type)

[imaNNeo]

The logic that we have in this method is a bit complicated. And it seems it silently ignores deprecated params when the current curve is not LineChartCubicTensionCurve.

I think we can have it simpler like this:

LineChartBarData copyWith({
  ...
}) =>
    LineChartBarData(
      ...
      curve: curve ?? _resolveCopyWithCurve(
        isCurved,
        curveSmoothness,
        preventCurveOverShooting,
        preventCurveOvershootingThreshold,
      ),
      ...
    );

LineChartCurve _resolveCopyWithCurve(
  bool? isCurved,
  double? curveSmoothness,
  bool? preventCurveOverShooting,
  double? preventCurveOvershootingThreshold,
) {
  final thisCurve = curve;
  if (isCurved == null) return thisCurve;
  if (!isCurved) return LineChartCurve.noCurve;
  if (thisCurve is LineChartCubicTensionCurve) {
    return LineChartCubicTensionCurve(
      smoothness: curveSmoothness ?? thisCurve.smoothness,
      preventCurveOverShooting:
      preventCurveOverShooting ?? thisCurve.preventCurveOverShooting,
      preventCurveOvershootingThreshold:
      preventCurveOvershootingThreshold ??
          thisCurve.preventCurveOvershootingThreshold,
    );
  }
  return LineChartCubicTensionCurve(
    smoothness: curveSmoothness ?? 0.35,
    preventCurveOverShooting: preventCurveOverShooting ?? false,
    preventCurveOvershootingThreshold: preventCurveOvershootingThreshold ?? 10.0,
  );
}

(I also double checked it with an LLM, and it seems it covers all the edge cases)

[imaNNeo]

And it would be very nice if you could write some unit tests that cover the backward compatibility for the constructor and copyWith functions. (For example, construct with deprecated params, and use copyWith using the new implementation, or the other way around)

Thanks!