iOS 12.5.7 cannot draw line gradient color

During testing on an iOS device running Version 12.5.7 (Build 16H81), we found that if the gradient is set to begin: Alignment.topCenter, end: Alignment.bottomCenter, and all spots have the same y-value, the line color cannot be drawn.

Screenshots

demo

import 'package:fl_chart/fl_chart.dart';
import 'package:flutter/material.dart';

class LineChartSample extends StatefulWidget {
  const LineChartSample({
    super.key,
    Color? gradientColor1,
    Color? gradientColor2,
    Color? gradientColor3,
  })  : gradientColor1 = gradientColor1 ?? Colors.blue,
        gradientColor2 = gradientColor2 ?? Colors.pink,
        gradientColor3 = gradientColor3 ?? Colors.orange;

  final Color gradientColor1;
  final Color gradientColor2;
  final Color gradientColor3;

  @override
  State<LineChartSample> createState() => _LineChartSampleState();
}

class _LineChartSampleState extends State<LineChartSample> {

  List<FlSpot> get allSpots => const [
        //works fine
        // FlSpot(0, 1),
        // FlSpot(1, 2),
        // FlSpot(2, 1.5),
        // FlSpot(3, 3),
        // FlSpot(4, 3.5),
        // FlSpot(5, 5),
        // FlSpot(6, 8),

        FlSpot(0, 5),
        FlSpot(1, 5),
        FlSpot(2, 5),
        FlSpot(3, 5),
        FlSpot(4, 5),
        FlSpot(5, 5),
        FlSpot(6, 5),
      ];

  @override
  Widget build(BuildContext context) {
    final lineBarsData = [
      LineChartBarData(
        spots: allSpots,
        isCurved: true,
        barWidth: 4,
        gradient: LinearGradient(
          begin: Alignment.topCenter,
          end: Alignment.bottomCenter,
          colors: [
            widget.gradientColor1,
            widget.gradientColor2,
            widget.gradientColor3,
          ],
          stops: const [0.1, 0.4, 0.9],
        ),
      ),
    ];

    return Scaffold(
      backgroundColor: Colors.white,
      body: Center(
        child: AspectRatio(
          aspectRatio: 1,
          child: LayoutBuilder(builder: (context, constraints) {
            return LineChart(
              LineChartData(
                lineBarsData: lineBarsData,
                minY: 0,
                maxY: 10,
                gridData: const FlGridData(show: false),
                borderData: FlBorderData(
                  show: true,
                  border: Border.all(
                    color: Colors.grey,
                  ),
                ),
              ),
            );
          }),
        ),
      ),
    );
  }
}

Versions

• Flutter: 3.22
• iOS:12.5.7 (Build 16H81)
• FlChart: 1.0.0

Start by running the supplied LineChartSample with the listed Flutter, fl_chart, and iOS versions, using the all-spots-equal-y reproduction. Trace the line gradient rendering entry point and compare it with the varying-y example. Done means the gradient is visible for the reported iOS case without regressing the working example.