LineChart visual error

**Describe the bug**
End of the line literally looks weird (not really being connected). Looks fine when [isCurved] is true, only happens when it's false.

**To Reproduce**
`import 'package:fl_chart/fl_chart.dart';
import 'package:flutter/material.dart';

void main() async {
return runApp(
MaterialApp(
home: Scaffold(
body: Center(
child: AspectRatio(
aspectRatio: 2,
child: LineChart(
LineChartData(
lineBarsData: [
LineChartBarData(
spots: [
FlSpot(1, 1),
FlSpot(2, 2),
FlSpot(3, 1),
FlSpot(4, 2),
FlSpot(5, 1),
],
isCurved: false,
curveSmoothness: 0.15,
preventCurveOverShooting: true,
dotData: FlDotData(
show: false,
),
),
],
),
),
),
),
),
),
);
}`

**Screenshots**

**Versions**
- Flutter 3.22.2
- fl_chart: 0.68.0

Start by running the provided LineChart reproduction with Flutter 3.22.2 and fl_chart 0.68.0, focusing on LineChartData and LineChartBarData when isCurved is false. Trace the LineChart rendering entry point to find why the final segment is not connected; done means the line end renders correctly while preserving the curved behavior.

Written by the indexing model from the issue text.