fix: LineChartDataPoint.tooltip not properly rendered (#5105)

* initial commit

* pubspec

Author: ndonkoHenri

packages/flet/lib/src/controls/linechart.dart

@@ -356,9 +356,7 @@ class _LineChartControlState extends State<LineChartControl> {
                       return touchedSpots.map((spot) {
                         var dp = viewModel.dataSeries[spot.barIndex]
                             .dataPoints[spot.spotIndex];
-                        var tooltip = dp.tooltip != null
-                            ? jsonDecode(dp.tooltip!)
-                            : dp.y.toString();
+                        var tooltip = dp.tooltip ?? dp.y.toString();
                         var tooltipStyle = parseTextStyle(
                             Theme.of(context), dp.control, "tooltipStyle");
                         tooltipStyle ??= const TextStyle();