1. A method comprising: storing in an electronic library a graphical skeleton component, the electronic library stored in a memory of a computing device, the graphical skeleton component having executable semantics and including a plurality of graphical objects, one or more computational relationships among the plurality of graphical objects, and at least one hole element, the at least one hole element indicating a location in the graphical skeleton component at which an expression is to be evaluated, the expression being unspecified, in response to a selection of the graphical skeleton component from the electronic library, creating, by a processor of the computing device coupled to the memory, an instance of the graphical skeleton component in a graphical model; converting the instance of the graphical skeleton component in the graphical model to a completed component by assigning a given expression to the at least one hole element of the instance of the graphical skeleton component; and at least one of executing the completed component within the graphical model, where executing of the completed component includes evaluating the given expression at the indicated location, or generating code for the completed component.