53. The apparatus of claim 49 wherein the graphical model executes over a plurality of simulation time steps, and at least one of the first graphical affordances is arranged on the display as being sent in a first of the simulation time steps and received in a second of the simulation time steps.
54. The apparatus of claim 53 wherein the lifelines are arranged vertically on the display, and the at least one of the first graphical affordances is arranged diagonally on the display.