diff --git a/core.go b/core.go
index 0e4345c..ccecd22 100644
--- a/core.go
+++ b/core.go
@@ -38,6 +38,8 @@ type TimeArrow struct {
 	ExpandTags  []string    `json:"expand_tags"`
 	ExpandValue interface{} `json:"expand_value"`
 	Weights     float64     `json:"weights"`
+  
+  	Extension map[string]interface{} `json:"extension"`
 }
 
 type TimeArrows []TimeArrow