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[Flutter/dart] Create a bar chart


Overview


You may want to display user data as a bar chart in your app. This time, I will introduce how to create a bar chart with flutter.


Method


We use a library called charts_flutter. (official)


Install


Add the following to pubsec.yaml.

charts_flutter: ^0.9.0

Add the following at the top of dart file.

import 'package:charts_flutter/flutter.dart' as charts;

Since the class name that defines the color of the graph called Color in charts_flutter overlaps with the existing Color, it is recommended to import it with a name.



Creating data for graphs


Graph the following sample class.

class SampleData{
  String name;
  int value;
  
  SampleData({this.name, this.value});
}

Converts a list of SampleData to data for a graph.

Widget makeChart(List<SampleData> datas){

  List<charts.Color> _colors= 
                           [charts.ColorUtil.fromDartColor(Colors.red), 
                            charts.Color.fromHex(code: "#1976D2"),
                            charts.MaterialPalette.green.shadeDefault, 
                            charts.Color.black];

  List<charts.Series<SampleData, String>> _series=[
    charts.Series<SampleData, String>(
      id: "sample",
      data: datas,
      domainFn: (_sample, _)=> _sample.name,
      measureFn: (_sample, _)=> _sample.value,
      colorFn: (_sample, i)
        => i<_colors.length? _colors[i] : _colors[_colors.length-1]
    )
  ];

domainFn is the method to determine the value of the x-axis and measureFn is the method to determine the value of the y-axis. The first argument is SampleData and the second argument is the index of the data.

colorFn is a method that determines the color of the graph. You can specify the color by converting from the Color class, specifying by code, or selecting from standard colors.


make chart

//continuation
  
  return Container(
    height: 400,
    child: charts.BarChart(
      _series,
      domainAxis: new charts.OrdinalAxisSpec(
        renderSpec: new charts.SmallTickRendererSpec(
          labelRotation: 45,  //1 rotate label
          labelStyle: new charts.TextStyleSpec(  //2 style of label
              fontSize: 16
            ),
          )
        ),
      ),
    );
  }

Pass the _series created above as the first argument of charts.BarChart.

After that, the axis label is set.

Part 1 is rotated 45 ° so that the x-axis labels do not overlap.

(Probably a bug, but if you set the rotation angle to 90 °, the height will be fixed)

Part 2 sets the format of the x-axis label.

There are many other settings you can make, so check the official site.


example


class MyHomePageState extends State<MyHomePage> {

  List<SampleData> datas;
  
  @override
  void initState() {
    datas=[
      SampleData(name: "Alice", value: 90),
      SampleData(name: "Bob", value: 85), 
      SampleData(name: "Charlie", value: 50),
      SampleData(name: "Daniel", value: 100)
    ];
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text('demo'),
        ),
        body: Container(
          padding: const EdgeInsets.all(16),
          child: makeChart(datas),
        )
    );
  }
  
  Widget makeChart(List<SampleData> datas){
    //略
  }
}

You can make the following graph!



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