Checking for existing displayedValue

Check if displayed value exist rather then if number evaluates to true. Enables the user to set the displayed value to zero.

templates/project/widgets/graph/graph.coffee

@@ -1,7 +1,7 @@
 class Dashing.Graph extends Dashing.Widget
 
   @accessor 'current', ->
-    return @get('displayedValue') if @get('displayedValue')
+    return @get('displayedValue') if @get('displayedValue')?
     points = @get('points')
     if points
       points[points.length - 1].y