Migrated the code with new charts.

Author: BPraveenBalu

lib/main.dart

@@ -62,7 +62,7 @@ class _MyHomePageState extends State<MyHomePage> {
                         primaryXAxis: CategoryAxis(),
                         // Chart title
                         title: ChartTitle(text: 'Half yearly sales analysis'),
-                        series: <ChartSeries<SalesData, String>>[
+                        series: <LineSeries<SalesData, String>>[
                           LineSeries<SalesData, String>(
                             dataSource: chartData,
                             xValueMapper: (SalesData sales, _) => sales.month,