Чтобы создать горизонтальную гистограмму с одной полосой во Flutter, вы можете использовать несколько методов. Вот несколько примеров:

Метод 1: использование пакета charts_flutter

import 'package:flutter/material.dart';
import 'package:charts_flutter/flutter.dart' as charts;
class SingleBarChart extends StatelessWidget {
  final List<charts.Series> seriesList;
  final bool animate;
  SingleBarChart(this.seriesList, {this.animate});
  factory SingleBarChart.withSampleData() {
    return SingleBarChart(
      _createSampleData(),
      animate: true,
    );
  }
  static List<charts.Series<OrdinalSales, String>> _createSampleData() {
    final data = [
      new OrdinalSales('Category 1', 5),
    ];
    return [
      new charts.Series<OrdinalSales, String>(
        id: 'Sales',
        colorFn: (_, __) => charts.MaterialPalette.blue.shadeDefault,
        domainFn: (OrdinalSales sales, _) => sales.category,
        measureFn: (OrdinalSales sales, _) => sales.sales,
        data: data,
      ),
    ];
  }
  @override
  Widget build(BuildContext context) {
    return charts.BarChart(
      seriesList,
      animate: animate,
    );
  }
}
class OrdinalSales {
  final String category;
  final int sales;
  OrdinalSales(this.category, this.sales);
}
void main() {
  runApp(
    MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Single Bar Chart'),
        ),
        body: Center(
          child: SingleBarChart.withSampleData(),
        ),
      ),
    ),
  );
}

Метод 2: использование пакета flutter_charts

import 'package:flutter/material.dart';
import 'package:flutter_charts/flutter_charts.dart';
class SingleBarChart extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Single Bar Chart'),
      ),
      body: Center(
        child: BarChart(
          data: [
            [5],
          ],
          maxValue: 10,
          barColors: [Colors.blue],
          xLabels: ['Category 1'],
          yLabels: [0, 5, 10],
          chartTitle: 'Sales',
          chartTitlePadding: 30,
          showLegend: false,
          barWidth: 40,
          showChartValue: true,
        ),
      ),
    );
  }
}
void main() {
  runApp(
    MaterialApp(
      home: SingleBarChart(),
    ),
  );
}