feat(dive_profile_chart): add used gas

lib/features/dive_log/data/services/gas_usage_segments_service.dart

@@ -0,0 +1,129 @@
+import 'package:submersion/features/dive_log/domain/entities/dive.dart';
+import 'package:submersion/features/dive_log/domain/entities/gas_switch.dart';
+
+/// One contiguous period during a dive when a single gas mix was breathed.
+///
+/// Drives the gas-usage timeline strip rendered below the dive profile.
+/// Times are in seconds from dive start; [endSeconds] is exclusive.
+class GasUsageSegment {
+  final int startSeconds;
+  final int endSeconds;
+  final GasMix gasMix;
+  final String label;
+  final String? tankName;
+
+  const GasUsageSegment({
+    required this.startSeconds,
+    required this.endSeconds,
+    required this.gasMix,
+    required this.label,
+    this.tankName,
+  });
+
+  int get durationSeconds => endSeconds - startSeconds;
+}
+
+/// Builds gas-usage segments for the dive profile gas-timeline strip.
+///
+/// Algorithm:
+/// 1. Pick the starting tank (lowest [DiveTank.order], or the first tank).
+/// 2. Sort gas switches by timestamp and clamp to dive bounds.
+/// 3. The initial segment runs from t=0 to the first switch's timestamp
+///    (skipped if the first switch is exactly at t=0).
+/// 4. Each subsequent segment runs from one switch to the next, ending at
+///    [diveDurationSeconds] for the final switch.
+/// 5. Adjacent segments with identical gas mixes are merged so a switch back
+///    to the same gas does not produce a visible seam.
+///
+/// Returns an empty list when there are no tanks or the dive has no
+/// duration — the caller should hide the strip in that case.
+List<GasUsageSegment> buildGasUsageSegments({
+  required List<DiveTank> tanks,
+  required List<GasSwitchWithTank> gasSwitches,
+  required int diveDurationSeconds,
+}) {
+  if (tanks.isEmpty || diveDurationSeconds <= 0) {
+    return const <GasUsageSegment>[];
+  }
+
+  final tankById = {for (final t in tanks) t.id: t};
+  final startingTank = ([
+    ...tanks,
+  ]..sort((a, b) => a.order.compareTo(b.order))).first;
+
+  final inBoundsSwitches =
+      ([...gasSwitches]..sort((a, b) => a.timestamp.compareTo(b.timestamp)))
+          .where((s) => s.timestamp >= 0 && s.timestamp <= diveDurationSeconds)
+          .toList(growable: false);
+
+  if (inBoundsSwitches.isEmpty) {
+    return [
+      GasUsageSegment(
+        startSeconds: 0,
+        endSeconds: diveDurationSeconds,
+        gasMix: startingTank.gasMix,
+        label: startingTank.gasMix.name,
+        tankName: startingTank.name,
+      ),
+    ];
+  }
+
+  final segments = <GasUsageSegment>[];
+
+  final firstSwitch = inBoundsSwitches.first;
+  if (firstSwitch.timestamp > 0) {
+    segments.add(
+      GasUsageSegment(
+        startSeconds: 0,
+        endSeconds: firstSwitch.timestamp,
+        gasMix: startingTank.gasMix,
+        label: startingTank.gasMix.name,
+        tankName: startingTank.name,
+      ),
+    );
+  }
+
+  for (var i = 0; i < inBoundsSwitches.length; i++) {
+    final cur = inBoundsSwitches[i];
+    final endSec = i + 1 < inBoundsSwitches.length
+        ? inBoundsSwitches[i + 1].timestamp
+        : diveDurationSeconds;
+    if (cur.timestamp >= endSec) continue;
+    final tank = tankById[cur.tankId];
+    final gasMix = GasMix(o2: cur.o2Fraction * 100, he: cur.heFraction * 100);
+    segments.add(
+      GasUsageSegment(
+        startSeconds: cur.timestamp,
+        endSeconds: endSec,
+        gasMix: gasMix,
+        label: gasMix.name,
+        tankName: tank?.name,
+      ),
+    );
+  }
+
+  return _mergeAdjacentSameGas(segments);
+}
+
+List<GasUsageSegment> _mergeAdjacentSameGas(List<GasUsageSegment> segments) {
+  if (segments.length < 2) return segments;
+  final merged = <GasUsageSegment>[segments.first];
+  for (var i = 1; i < segments.length; i++) {
+    final last = merged.last;
+    final cur = segments[i];
+    final sameGas =
+        last.gasMix.o2 == cur.gasMix.o2 && last.gasMix.he == cur.gasMix.he;
+    if (sameGas && last.endSeconds == cur.startSeconds) {
+      merged[merged.length - 1] = GasUsageSegment(
+        startSeconds: last.startSeconds,
+        endSeconds: cur.endSeconds,
+        gasMix: last.gasMix,
+        label: last.label,
+        tankName: last.tankName,
+      );
+    } else {
+      merged.add(cur);
+    }
+  }
+  return merged;
+}

lib/features/dive_log/domain/entities/dive.dart

@@ -918,10 +918,12 @@ class GasMix extends Equatable {
   bool get isAir => o2 >= 20 && o2 <= 22 && he == 0;
   bool get isNitrox => o2 > 22 && he == 0;
   bool get isTrimix => he > 0;
+  bool get isOxygen => o2 >= 99 && he == 0;
 
   String get name {
     if (isAir) return 'Air';
     if (isTrimix) return 'Tx $roundedO2/$roundedHe';
+    if (isOxygen) return 'O2';
     if (isNitrox) return 'EAN$roundedO2';
     return '$roundedO2% O2';
   }

lib/features/dive_log/domain/entities/gas_switch.dart

@@ -108,6 +108,9 @@ class GasSwitchWithTank extends Equatable {
   /// Whether this is air
   bool get isAir => o2Fraction >= 0.20 && o2Fraction <= 0.22 && heFraction == 0;
 
+  /// Whether this is pure oxygen (used as deco gas)
+  bool get isOxygen => o2Fraction >= 0.99 && heFraction == 0;
+
   /// ppO2 at switch depth
   double get ppO2AtDepth {
     if (gasSwitch.depth == null) return o2Fraction;

lib/features/dive_log/presentation/pages/dive_detail_page.dart

@@ -28,6 +28,7 @@ import 'package:submersion/features/marine_life/domain/entities/species.dart';
 import 'package:submersion/features/marine_life/presentation/providers/species_providers.dart';
 import 'package:submersion/features/settings/presentation/providers/export_providers.dart';
 import 'package:submersion/features/settings/presentation/providers/settings_providers.dart';
+import 'package:submersion/features/dive_log/data/services/gas_usage_segments_service.dart';
 import 'package:submersion/features/dive_log/data/services/profile_analysis_service.dart';
 import 'package:submersion/features/dive_log/data/services/profile_markers_service.dart';
 import 'package:submersion/features/dive_log/domain/entities/cylinder_sac.dart';
@@ -1289,6 +1290,19 @@ class _DiveDetailPageState extends ConsumerState<DiveDetailPage> {
                         tanks: dive.tanks,
                         tankPressures: tankPressures,
                         gasSwitches: gasSwitchesAsync.valueOrNull,
+                        gasSegments:
+                            (dive.tanks.isEmpty || dive.profile.isEmpty)
+                            ? null
+                            : buildGasUsageSegments(
+                                tanks: dive.tanks,
+                                gasSwitches:
+                                    gasSwitchesAsync.valueOrNull ?? const [],
+                                diveDurationSeconds:
+                                    dive.profile.last.timestamp,
+                              ),
+                        diveDurationSeconds: dive.profile.isEmpty
+                            ? null
+                            : dive.profile.last.timestamp,
                         computerProfiles: multiComputerProfiles,
                         visibleComputers: effectiveVisible,
                         computerLineColors: computerLineColors,
@@ -4943,6 +4957,21 @@ class _FullscreenProfilePageState
                       tanks: dive.tanks,
                       tankPressures: widget.tankPressures,
                       gasSwitches: widget.gasSwitches,
+                      gasSegments: (dive.tanks.isEmpty || dive.profile.isEmpty)
+                          ? null
+                          : buildGasUsageSegments(
+                              tanks: dive.tanks,
+                              gasSwitches: widget.gasSwitches ?? const [],
+                              diveDurationSeconds: dive.profile.last.timestamp,
+                            ),
+                      diveDurationSeconds: dive.profile.isEmpty
+                          ? null
+                          : dive.profile.last.timestamp,
+                      highlightedTimestamp:
+                          _selectedPointIndex != null &&
+                              _selectedPointIndex! < dive.profile.length
+                          ? dive.profile[_selectedPointIndex!].timestamp
+                          : null,
                       onPointSelected: (index) {
                         setState(() {
                           _selectedPoint = index != null

lib/features/dive_log/presentation/providers/gas_switch_providers.dart

@@ -12,12 +12,13 @@ final gasSwitchesProvider =
     });
 
 /// Gas type classification for coloring
-enum GasType { air, nitrox, trimix }
+enum GasType { air, nitrox, oxygen, trimix }
 
 /// Extension to determine gas type from GasSwitchWithTank
 extension GasSwitchWithTankGasType on GasSwitchWithTank {
   GasType get gasType {
     if (isTrimix) return GasType.trimix;
+    if (isOxygen) return GasType.oxygen;
     if (isNitrox) return GasType.nitrox;
     return GasType.air;
   }
@@ -27,6 +28,7 @@ extension GasSwitchWithTankGasType on GasSwitchWithTank {
 extension GasTypeFromFractions on ({double o2, double he}) {
   GasType get gasType {
     if (he > 0) return GasType.trimix;
+    if (o2 >= 0.99) return GasType.oxygen;
     if (o2 > 0.22) return GasType.nitrox;
     return GasType.air;
   }

lib/features/dive_log/presentation/providers/profile_legend_provider.dart

@@ -55,6 +55,9 @@ class ProfileLegendState {
   // Per-tank pressure visibility (keyed by tank ID)
   final Map<String, bool> showTankPressure;
 
+  // Gas timeline strip visibility
+  final bool showGas;
+
   // Collapsible section expanded/collapsed state (session-only)
   final Map<String, bool> sectionExpanded;
 
@@ -88,6 +91,7 @@ class ProfileLegendState {
     this.ttsSource = MetricDataSource.calculated,
     this.cnsSource = MetricDataSource.calculated,
     this.showTankPressure = const {},
+    this.showGas = true,
     this.sectionExpanded = const {
       'overlays': true,
       'decompression': true,
@@ -159,6 +163,7 @@ class ProfileLegendState {
     MetricDataSource? ttsSource,
     MetricDataSource? cnsSource,
     Map<String, bool>? showTankPressure,
+    bool? showGas,
     Map<String, bool>? sectionExpanded,
   }) {
     return ProfileLegendState(
@@ -193,6 +198,7 @@ class ProfileLegendState {
       ttsSource: ttsSource ?? this.ttsSource,
       cnsSource: cnsSource ?? this.cnsSource,
       showTankPressure: showTankPressure ?? this.showTankPressure,
+      showGas: showGas ?? this.showGas,
       sectionExpanded: sectionExpanded ?? this.sectionExpanded,
     );
   }
@@ -231,6 +237,7 @@ class ProfileLegendState {
           ttsSource == other.ttsSource &&
           cnsSource == other.cnsSource &&
           mapEquals(showTankPressure, other.showTankPressure) &&
+          showGas == other.showGas &&
           mapEquals(sectionExpanded, other.sectionExpanded);
 
   @override
@@ -264,6 +271,7 @@ class ProfileLegendState {
     ttsSource,
     cnsSource,
     ...showTankPressure.entries,
+    showGas,
     ...sectionExpanded.entries,
   ]);
 }
@@ -458,6 +466,11 @@ class ProfileLegend extends _$ProfileLegend {
     );
   }
 
+  /// Toggle visibility of the gas-usage timeline strip below the chart.
+  void toggleGas() {
+    state = state.copyWith(showGas: !state.showGas);
+  }
+
   /// Toggle visibility for a specific tank's pressure line
   void toggleTankPressure(String tankId) {
     final current = state.showTankPressure[tankId] ?? true;