Touch Callback Line Touch Response Expected Result

Hi, thanks for an amazing package. I want to make sure that the line touch response of x axis (`touchResponse?.lineBarSpots?.first.x`) is expected to return an index of bar tapped whenever it is just we tap on certain bar point? Because when I just drag and remove my finger on a certain point it will return null.

Here is the recording bug:

https://github.com/imaNNeo/fl_chart/assets/110516419/c8de7471-5f87-411d-8e1c-4411335376c1

I deep down into `FlTouchEvent` class and find that the event is different (removed **touch** is `FlTapUpEvent` and removed **drag** is `FlPanCancelEvent`) but still get confused about the return index from those events regarding the UX that maybe for many users are expect the same result (returning null whenever we **remove** our finger over the chart)

- Fl_chart version `0.63.0`
- Flutter SDK version `3.10.0`

Thank you 🙌🏻

Reproduce the reported touch sequence from the linked recording using fl_chart 0.63.0 and Flutter 3.10.0; start by tracing FlTouchEvent handling, especially FlTapUpEvent versus FlPanCancelEvent. Compare lineBarSpots?.first.x for tap and drag-release, then define and verify the expected result for each interaction.

Written by the indexing model from the issue text.