diff --git a/timeline-chart/src/time-graph-model.ts b/timeline-chart/src/time-graph-model.ts
index 0dd2cba..68e7bcd 100644
--- a/timeline-chart/src/time-graph-model.ts
+++ b/timeline-chart/src/time-graph-model.ts
@@ -28,7 +28,7 @@ export namespace TimelineChart {
     export interface TimeGraphState {
         readonly id: string
         readonly range: TimeGraphRange
-        readonly label: string
+        readonly label?: string
         selected?: boolean
         readonly data?: { [key: string]: any }
     }