use a Maybe instead of NoTarget

frontend/src/Main.elm

@@ -145,7 +145,6 @@ handleDragFinish model target (x, y) =
                       (s, d) = model.markedTime
                   in (s, d - deltat)
             }
-        NoTarget -> model
 
 
 updateModel msg model =
@@ -347,12 +346,12 @@ targetedEventDecoder =
         Pointer.eventDecoder
         (D.at ["target", "id"] D.string)
 
-targetFor : String ->  DragTarget
+targetFor : String -> Maybe DragTarget
 targetFor s =
     case s of
-        "left-marker" -> StartMark
-        "right-marker" -> EndMark
-        _ -> NoTarget
+        "left-marker" -> Just StartMark
+        "right-marker" -> Just EndMark
+        _ -> Nothing
 
 onDownWithTarget tag =
     let
@@ -439,7 +438,11 @@ timeAxis model points =
         svg
         [ width portalWidth
         , height (graphHeight + 20)
-        , onDownWithTarget (\e -> DragStart (targetFor e.targetId) (epos e.pointerEvent))
+        , onDownWithTarget (\e ->
+                                case targetFor e.targetId of
+                                    Just tgt -> DragStart tgt (epos e.pointerEvent)
+                                    Nothing -> Dribble "drag with unknown target"
+                           )
         , viewBox ("0 -10 " ++ (String.fromInt portalWidth) ++
                        "  " ++ (String.fromInt (graphHeight + 10)))
         ]

frontend/src/Model.elm

@@ -8,7 +8,7 @@ module Model exposing
 import TileMap exposing (ZoomLevel, Coord)
 import Point exposing (Point)
 
-type DragTarget = Map | Graph | StartMark | EndMark | NoTarget
+type DragTarget = Map | Graph | StartMark | EndMark
 
 type Drag
     = None