diff --git a/lib/xmet/series.py b/lib/xmet/series.py
index 6e502b7..b8c8a9c 100644
--- a/lib/xmet/series.py
+++ b/lib/xmet/series.py
@@ -13,13 +13,17 @@ class Series(dict):
     def __init__(self):
         super().__init__()
 
-    def intersect(self, series: Self, intersection: SeriesIntersection) -> tuple[float]:
+    def intersect(self, series: Self, intersection: SeriesIntersection, start: float=None) -> tuple[float]:
         pairs = nearest(sorted(self.keys(),   reverse=True),
                         sorted(series.keys(), reverse=True))
 
         for pair in pairs:
             v1, v2 = self[pair[0]], series[pair[1]]
 
+            if start is not None:
+                if pair[0] > start:
+                    continue
+
             sign = cmp(v1, v2)
 
             if sign == intersection.value: