late TooltipBehavior _tooltipBehavior;

    @override
    void initState(){
      _tooltipBehavior =  TooltipBehavior(enable: true);
      super.initState(); 
    }
    @override
    Widget build(BuildContext context) {
        return Scaffold(
            appBar: AppBar(
                title: Text(widget.title),
            ),
            body: Center(
                child: Container(
                    child: SfCircularChart(
                        // Enables the tooltip for all the series in chart
                        tooltipBehavior: _tooltipBehavior,
                        series: <ChartSeries>[
                            // Initialize line series
                            PieSeries<SalesData, String>(
                                // Enables the tooltip for individual series
                                enableTooltip: true, 
                                dataSource: [
                                    // Bind data source
                                    SalesData('Jan', 35),
                                    SalesData('Feb', 28),
                                    SalesData('Mar', 34),
                                    SalesData('Apr', 32),
                                    SalesData('May', 40)
                                ],
                                xValueMapper: (SalesData sales, _) => sales.year,
                                yValueMapper: (SalesData sales, _) => sales.sales
                            )
                        ]
                    )
                )
            )
        );
    }