Opam_graph: Added visualization-version number

src/opam_graph.ml

@@ -1,4 +1,12 @@
 
+let visualization_version = 1
+(** Remember to increment this when anything changes that can affect the 
+    visualization, e.g.:
+      * algorithm change
+      * UI change
+      * certain library-dependency changes 
+*)
+
 let sprintf = Printf.sprintf
 
 module OSet = OpamPackage.Set