add graph_view

lib/models/chart_model.dart

@@ -0,0 +1,62 @@
+class ChartDataPoint {
+  final int stopNumber;
+  final double litersPerStop;
+  final double pricePerStop;
+  final double pricePerLiter;
+
+  ChartDataPoint({
+    required this.stopNumber,
+    required this.litersPerStop,
+    required this.pricePerStop,
+    required this.pricePerLiter,
+  });
+}
+
+class ChartData {
+  final List<ChartDataPoint> dataPoints;
+
+  ChartData({required this.dataPoints});
+
+  factory ChartData.fromTankList(List<dynamic> tankList) {
+    List<ChartDataPoint> points = [];
+
+    for (int i = 0; i < tankList.length; i++) {
+      final tank = tankList[i];
+      final liters = double.tryParse(tank.szLiters) ?? 0.0;
+      final priceTotal = double.tryParse(tank.szPriceTotal) ?? 0.0;
+      final pricePerLiter = double.tryParse(tank.szPricePerLiter) ?? 0.0;
+
+      points.add(
+        ChartDataPoint(
+          stopNumber: i + 1,
+          litersPerStop: liters,
+          pricePerStop: priceTotal,
+          pricePerLiter: pricePerLiter,
+        ),
+      );
+    }
+
+    return ChartData(dataPoints: points);
+  }
+
+  double getMaxLiters() {
+    if (dataPoints.isEmpty) return 0;
+    return dataPoints
+        .map((p) => p.litersPerStop)
+        .reduce((a, b) => a > b ? a : b);
+  }
+
+  double getMaxPrice() {
+    if (dataPoints.isEmpty) return 0;
+    return dataPoints
+        .map((p) => p.pricePerStop)
+        .reduce((a, b) => a > b ? a : b);
+  }
+
+  double getMaxPricePerLiter() {
+    if (dataPoints.isEmpty) return 0;
+    return dataPoints
+        .map((p) => p.pricePerLiter)
+        .reduce((a, b) => a > b ? a : b);
+  }
+}