【flutter】fl_chartで横軸が日付の線グラフを作成したい

Question

前提

flutterで線グラフを作成しようとしています

実現したいこと

本日の日付を横軸の一番右した線グラフを作成したいです

発生している問題・エラーメッセージ

下記4つのエラーが出ています

In a const constructor, a value of type 'Null' can't be assigned to the field 'x', which has type 'double'.  Try using a subtype, or removing the keyword 'const'.

A value of type 'Null' can't be assigned to a parameter of type 'double' in a const constructor. (Documentation)  Try using a subtype, or removing the keyword 'const'.

Arguments of a constant creation must be constant expressions. (Documentation)  Try making the argument a valid constant, or use 'new' to call the constructor.

The values in a const list literal must be constants. (Documentation)  Try removing the keyword 'const' from the list literal.

該当のソースコード

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

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(

        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key? key, required this.title}) : super(key: key);

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {

  DateTime a = DateTime.now();
  String b = DateTime.now().toString();
  String c = DateTime.now().toString().substring(8,10);
  double daysubstring = double.parse(DateTime.now().toString().substring(8,10));

  @override
  Widget build(BuildContext context) {

    return Scaffold(
      appBar: AppBar(

        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Container(
              padding: const EdgeInsets.only(left: 8, right: 32),
              height: 300,
              width: double.infinity,
              child: LineChart(LineChartData(
                  backgroundColor: Colors.grey[200],
                  // titlesData: _titles,
                  minX: 1,
                  maxX: 12,
                  minY: 0,
                  maxY: 30,
                  lineBarsData: [
                    LineChartBarData(
                      isCurved: true,
                      color: Colors.red[400],
                      barWidth: 4,
                      dotData: FlDotData(show: true),
                      spots: const [
                        FlSpot(1, 5.4),
                        FlSpot(2, 8.5),
                        FlSpot(3, 12.8),
                        FlSpot(4, 15.1),
                        FlSpot(5, 19.6),
                        FlSpot(6, 22.7),
                        FlSpot(7, 25.9),
                        FlSpot(8, 27.4),
                        FlSpot(9, 22.3),
                        FlSpot(10, 18.2),
                        FlSpot(11, 13.7),
                        FlSpot(daysubstring, 7.9),
                      ],
                    ),
                  ])),
            ),
          ],
        ),
      ),
    );
  }
}

補足情報（FW/ツールのバージョンなど）

ここにより詳細な情報を記載してください。

Answer 1

ちょっと何がゴールかはっきりわからないのですが、
とりあえずエラーを出なくしたいのなら下記のように変更して結果をご確認ください。
daysubstringはコンパイル時定数(const定数)ではないので、constをつけたListの要素にはできないです。

spots: //const 
                      [
                        FlSpot(1, 5.4),
                        FlSpot(2, 8.5),
                        FlSpot(3, 12.8),
                        FlSpot(4, 15.1),
                        FlSpot(5, 19.6),
                        FlSpot(6, 22.7),
                        FlSpot(7, 25.9),
                        FlSpot(8, 27.4),
                        FlSpot(9, 22.3),
                        FlSpot(10, 18.2),
                        FlSpot(11, 13.7),
                        FlSpot(daysubstring, 7.9),
                      ],