DIAGRAM: small refactor

Enhanced the Diag type and used it for poly functions as well.

Graphics/Diagram/Types.hs

@@ -16,11 +16,15 @@ type MeshString = String
 -- coordinates and common properties.
 data Diag = Diag {
   mkDiag :: DiagProp
-         -> [PT]
+         -> Object
          -> Diagram Cairo R2
 }
 
 
+data Object = Object [PT]
+            | Objects [[PT]]
+
+
 -- |Holds the properties for a Diagram, like thickness of 2d points etc.
 -- This can also be seen as a context when merging multiple diagrams.
 data DiagProp = MkProp {