Update LineChart interval example

main.dart

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

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({super.key});

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  double horizontalInterval = 1.0;

  double verticalInterval = 1.0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: SizedBox(
          width: 500,
          child: Column(
            mainAxisSize: MainAxisSize.min,
            children: [
              SizedBox(
                height: 300,
                child: Row(
                  children: [
                    Column(
                      mainAxisSize: MainAxisSize.min,
                      children: [
                        Text(
                          '$horizontalInterval',
                          style: GoogleFonts.roboto(
                            fontSize: 16,
                            fontWeight: FontWeight.bold,
                          ),
                        ),
                        Expanded(
                          child: RotatedBox(
                            quarterTurns: 3,
                            child: SizedBox(
                              width: 20,
                              height: 40,
                              child: Slider(
                                value: horizontalInterval,
                                divisions: 8,
                                onChanged: (newValue) {
                                  setState(() {
                                    horizontalInterval = newValue;
                                  });
                                },
                                min: 1,
                                max: 5,
                              ),
                            ),
                          ),
                        ),
                      ],
                    ),
                    Expanded(
                      child: _MyChart(
                        horizontalInterval: horizontalInterval,
                        verticalInterval: verticalInterval,
                      ),
                    ),
                  ],
                ),
              ),
              Row(
                children: [
                  Expanded(
                    child: Slider(
                      value: verticalInterval,
                      onChanged: (newValue) {
                        setState(() {
                          verticalInterval = newValue;
                        });
                      },
                      min: 1,
                      max: 5,
                      divisions: 8,
                    ),
                  ),
                  Text(
                    '$verticalInterval',
                    style: GoogleFonts.roboto(
                      fontSize: 16,
                      fontWeight: FontWeight.bold,
                    ),
                  ),
                ],
              ),
            ],
          ),
        ),
      ),
    );
  }
}

class _MyChart extends StatelessWidget {
  const _MyChart({
    required this.horizontalInterval,
    required this.verticalInterval,
  });

  final double horizontalInterval;
  final double verticalInterval;

  @override
  Widget build(BuildContext context) {
    return LineChart(
      LineChartData(
        lineBarsData: [
          LineChartBarData(
            spots: const [
              FlSpot(0, 1),
              FlSpot(1, 2),
              FlSpot(2, 3),
              FlSpot(3, 2),
              FlSpot(4, 5),
              FlSpot(5, 3),
              FlSpot(6, 6),
              FlSpot(7, 4),
              FlSpot(8, 5),
              FlSpot(9, 9),
            ],
          ),
        ],
        titlesData: FlTitlesData(
          bottomTitles: AxisTitles(
            sideTitles: SideTitles(
              reservedSize: 26,
              showTitles: true,
              interval: verticalInterval,
            ),
          ),
          topTitles: AxisTitles(
            sideTitles: SideTitles(
              reservedSize: 26,
              showTitles: true,
              interval: verticalInterval,
            ),
          ),
          leftTitles: AxisTitles(
            sideTitles: SideTitles(
              reservedSize: 40,
              showTitles: true,
              interval: horizontalInterval,
            ),
          ),
          rightTitles: AxisTitles(
            sideTitles: SideTitles(
              reservedSize: 40,
              showTitles: true,
              interval: horizontalInterval,
            ),
          ),
        ),
        gridData: FlGridData(
          horizontalInterval: horizontalInterval,
          verticalInterval: verticalInterval,
        ),
      ),
    );
  }
}

Answered in here:
imaNNeo/fl_chart#1691