1. A computer accessible memory medium comprising program instructions executable by a processor to: display on a display a first diagram, wherein the first diagram represents a physical system and is executable to simulate the physical system, wherein the first diagram comprises a plurality of components corresponding to components of the physical system, wherein the plurality of components are interconnected by lines, wherein the lines interconnecting the plurality of components in the first diagram comprise lines of a first type, and wherein the lines of the first type represent physical connections among the components in the physical system; display on the display a graphical data flow diagram connected to the first diagram, wherein the graphical data flow diagram comprises a plurality of interconnected blocks that visually indicate functionality of the graphical data flow diagram, wherein the blocks in the graphical data flow diagram are interconnected by lines of a second type, and wherein the lines of the second type represent data flow semantics among the blocks in the graphical data flow diagram; execute the first diagram to simulate the physical system; and execute the graphical data flow diagram, wherein said executing the graphical data flow diagram comprises one or more of: passing a first simulation value computed by the simulation of the physical system as input to the graphical data flow diagram; or producing output data, wherein the output data is provided as an input signal for the simulation of the physical system.