always set lane

packages/z-timeline/lib/state.dart

@@ -61,6 +61,7 @@ class TimelineItemData {
   final String? description;
   final String start;
   final String? end;
+  final int lane;
 
   TimelineItemData({
     required this.id,
@@ -68,6 +69,7 @@ class TimelineItemData {
     this.description,
     required this.start,
     this.end,
+    required this.lane,
   });
 
   factory TimelineItemData.fromJson(Map<String, dynamic> json) {
@@ -77,6 +79,7 @@ class TimelineItemData {
       description: json['description'] as String?,
       start: json['start'] as String,
       end: json['end'] as String?,
+      lane: json['lane'] as int,
     );
   }
 }