I generally have given up on diagrams. Systems and flows I work with are too convoluted to be mapped out. Only the simplest of flows can be diagrammed and it usually leaves out important facts. When dealing with non technical people, I have found out through trial and error that excel works best. I start out with sample data on one sheet and walk them through the various transformations in sheet2, sheet3 etc. I even create a table of contents that has links to the different sheets. In a phrase, seeing data is believing.