title: Navigate from and to the Donut chart gets blank using charts_flutter in flutter
body: <p>I am using the donut chart using the package <strong>charts_flutter V0.8.1</strong>.  But when I navigate to the next screen and get back to the previous page(Donut page) chart gets blank. Scenario: Once I can see the next screen I pressed the back button. FYI I am using the Donut chart sample code.<a href="https://google.github.io/charts/flutter/example/pie_charts/donut" rel="nofollow noreferrer">https://google.github.io/charts/flutter/example/pie_charts/donut</a></p>
  
  <pre><code>import 'package:charts_flutter/flutter.dart' as charts;
  import 'package:flutter/material.dart';
  void main() {
    runApp(MaterialApp(
        debugShowCheckedModeBanner: false,
        home:  new DonutPieChart()
        ));
  }
  class DonutPieChart extends StatefulWidget {
    @override
    _DonutPieChartState createState() =&gt; new _DonutPieChartState();
  }
  
  class _DonutPieChartState extends State&lt;DonutPieChart&gt; {
    List&lt;charts.Series&lt;Task, String&gt;&gt; _seriesPieData;
  
    @override
    void initState() {
      super.initState();
      _seriesPieData = List&lt;charts.Series&lt;Task, String&gt;&gt;();
      _generateData();
    }
  
    _generateData() {
      var piedata = [
        new Task('T01', 35, Color(0xff3366cc)),
        new Task('T02', 8, Color(0xff990099)),
        new Task('T03', 10, Color(0xff109618)),
      ];
  
      _seriesPieData.add(
        charts.Series(
          domainFn: (Task task, _) =&gt; task.task,
          measureFn: (Task task, _) =&gt; task.taskvalue,
          colorFn: (Task task, _) =&gt;
              charts.ColorUtil.fromDartColor(task.colorval),
          id: 'Sample',
          data: piedata,
          labelAccessorFn: (Task row, _) =&gt; '${row.taskvalue}',
        ),
      );
    }
  
    @override
    Widget build(BuildContext _context) {
      return Scaffold(
          appBar: AppBar(
            title: Text('Donut chart'),
          ),
          body: GestureDetector(
            onTap: () {
              Navigator.of(_context).push(new MaterialPageRoute&lt;bool&gt;(
                  builder: (BuildContext context) =&gt; NextScreen()));
            },
            child: Column(
              children: &lt;Widget&gt;[
                Expanded(
                  child: Container(
                    alignment: Alignment.center,
                    height: 230,
                    width: 280,
                    child: charts.PieChart(_seriesPieData,
                        animate: true,
                        animationDuration: Duration(seconds: 1),
                        defaultInteractions: false,
                        behaviors: [
                          new charts.DatumLegend(
                            outsideJustification:
                                charts.OutsideJustification.endDrawArea,
                            horizontalFirst: false,
                            desiredMaxRows: 3,
                            cellPadding: new EdgeInsets.only(
                                left: 8, right: 4.0, bottom: 4.0),
                            entryTextStyle: charts.TextStyleSpec(
                                color: charts.Color.fromHex(code: "#000000"),
                                fontFamily: 'Georgia',
                                fontSize: 15),
                          )
                        ],
                        defaultRenderer: new charts.ArcRendererConfig(
                            arcWidth: 30,
                            strokeWidthPx: 0,
                            arcRendererDecorators: [
                              new charts.ArcLabelDecorator(
                                  insideLabelStyleSpec: new charts.TextStyleSpec(
                                      fontSize: 16,
                                      color:
                                          charts.Color.fromHex(code: "#000000")),
                                  labelPosition: charts.ArcLabelPosition.inside)
                            ])),
                  ),
                ),
              ],
            ),
          ));
    }
  }
  
  class Task {
    String task;
    double taskvalue;
    Color colorval;
    Task(this.task, this.taskvalue, this.colorval);
  }
  
  class NextScreen extends StatefulWidget {
    @override
    _NextScreenState createState() =&gt; new _NextScreenState();
  }
  
  class _NextScreenState extends State&lt;NextScreen&gt; {
    @override
    Widget build(BuildContext context) {
      return new Scaffold(
        appBar: AppBar(title: Text(' ')),
      );
    }
  }
  
  </code></pre>