Line chart appear animation

[Michal-MK]

Possibly closing #1223 as we also had this issue in our project so I implemented this and now ported it to newest fl_chart to see if it can be merged.

2025-09-10.23-13-03.mp4

So far no tests and no coverage stuff (I read the contributors guide :D)

If this is accepted as a solution I will look at that and address it in the PR.

[imaNNeo]

Hi, thanks for your PR and for implementing the loading animation.
Sorry for my late reply.

To be honest, I don't think this solution is scalable and customizable.
I would like to implement it in a way that we can scale it, and users are able to customize it.
These are the comments I got from my LLM:

1. LineChartPaintHolder and LineChartEntryAnimation are line-chart-specific, but they're
   used in base classes like AxisChartPainter and AxisChartScaffoldWidget that are shared
   across all chart types. Let's keep the base layer chart-type-agnostic — you can move these
   to the line chart package and use polymorphism (e.g. override a getter) instead of the if
   (holder is LineChartPaintHolder) type check.
2. The clipping logic is repeated 4 times in drawBarLine, drawDots, drawBelowBarPath, and
   drawAboveBarPath. Let's extract it into a helper.
3. dependency_overrides for vector_math shouldn't be in a published library's pubspec.yaml —
   it can cause issues for downstream users.
4. The naming is inconsistent — the same concept is called LineChartEntryAnimation,
   animationType, appearAnimationType, and entryAnimation in different places. Let's pick one
   name.
5. No tests for the actual animation behavior (clipping at different progress values,
   targetData usage during slide, dots hidden in leftToRightSlideWithoutPoints). The current
   test changes are just mechanical PaintHolder → LineChartPaintHolder updates.
6. The enum values bake "leftToRight" into the name, which would make adding other
   directions awkward later. A config object separating direction from content options might
   work better — but just a thought, happy to discuss.

And now there are a lot of conflicts. So I'll close it for now.

Thanks again!

[Michal-MK]

Oh hey this PR existed :D No worries, maybe you can take something from it and implement it better.

[imaNNeo]

Haha!
Sorry, sometimes I have to do this.
But I'll keep this feature in my mind ot implement asap. We should find a simpler and more scalable solution.

Thanks for your contribution btw, it definitely gives some idea to me (and my LLM :)