add graph_view

lib/controller/graph_controller.dart

@@ -0,0 +1,60 @@
+import 'package:flutter_tank_web_app/models/tank_model.dart';
+import 'package:get/get.dart';
+
+class GraphController extends GetxController {
+  final listTankModel = <TankModel>[].obs;
+  final sumYearKm = 0.0.obs;
+  final sumYearLiters = 0.0.obs;
+  final sumYearPrice = 0.0.obs;
+  final averagePricePerLiter = 0.0.obs;
+
+  @override
+  void onInit() {
+    if (Get.arguments != null) {
+      var args = Get.arguments as List<TankModel>;
+      listTankModel.addAll(args);
+      _calcSumms();
+    }
+    super.onInit();
+  }
+
+  @override
+  void onReady() {}
+
+  @override
+  void onClose() {}
+
+  // Calculate yearly summaries
+  void _calcSumms() {
+    if (listTankModel.length < 2) {
+      sumYearKm.value = 0.0;
+      sumYearLiters.value = 0.0;
+      sumYearPrice.value = 0.0;
+      return;
+    }
+
+    // Sort by date to calculate km correctly
+    listTankModel.sort((a, b) {
+      var dateA = DateTime.tryParse(a.szDate)!;
+      var dateB = DateTime.tryParse(b.szDate)!;
+      return dateA.compareTo(dateB);
+    });
+
+    var firstOdometer = double.tryParse(listTankModel.first.szOdometer) ?? 0.0;
+    var lastOdometer = double.tryParse(listTankModel.last.szOdometer) ?? 0.0;
+
+    sumYearKm.value = lastOdometer - firstOdometer;
+
+    double totalLiters = 0.0;
+    double totalPrice = 0.0;
+
+    for (var tank in listTankModel) {
+      totalLiters += double.tryParse(tank.szLiters) ?? 0.0;
+      totalPrice += double.tryParse(tank.szPriceTotal) ?? 0.0;
+    }
+
+    sumYearLiters.value = totalLiters;
+    sumYearPrice.value = totalPrice;
+    averagePricePerLiter.value = totalPrice / totalLiters;
+  }
+}