A "graph" usually means an X-Y plot, using Cartesian coordinates. A "diagram" means any visual presentation that is intended to explain or show a logical idea--categories, relationships, cause-and-effect, quantitative relationships. A diagram is intended to show how something works, not how it it looks. A graph is one kind of diagram. Yes, the words are related. "Graph," and the "-gram" part of "diagram," both come from the Greek word meaning "to write." This is a graph: http://radioopensource.org/wp-content/uploads/2014/04/Graph-5.png This is a diagram: https://www.tutorialspoint.com/uml/images/uml_component_diagram.jpg This is another diagram: https://i.stack.imgur.com/2OT3B.png This is another diagram: http://tinyurl.com/y7rqaku7

A "diagram" can be any type of graphical representation of a thing. Think of the pictures/drawings you will see giving basic instructions (like on hand dryers in the restrooms around here). A graph could be considered a type of diagram, but graphs are used to show, track, or compare values (measurements) of different things. Hopefully that makes sense?