diff --git a/frontend/src/Main.elm b/frontend/src/Main.elm
index 9b9e012..971bbfc 100644
--- a/frontend/src/Main.elm
+++ b/frontend/src/Main.elm
@@ -113,8 +113,12 @@ dragUpdate model (newx, newy) =
           in { model | centre = translate fromcoord (pixelsToCoord model.zoom t) }
       Just (DragGraph (fromx,_) fromtime) ->
           let time = secondsFromPixels model (fromx - newx)
+              startTime = fromtime + time
+              clampVisible = clamp startTime (startTime + model.duration)
           in { model |
-                   startTime = fromtime + time
+                   startTime = startTime
+                 , leftMark = clampVisible model.leftMark
+                 , rightMark = clampVisible model.rightMark
              }
       Just (DragLeftMark (fromx,_) fromtime) ->
           let time = secondsFromPixels model (fromx - newx)