Thanks for getting in touch. The diagramming features are a bit fiddly, they do require a little bit of trial and error to understand how drag and drop works and how to draw lines between elements. On the other hand, they do allow you to collect up to date (and auto update) numbers from your coding trees, so what you're trying to achieve should be possible.
Did you have a look at the Manual? The relative section (from page 123) isn't the most detailed, but I do think it should be sufficient to get you started and it does mention how to move elements around (I personally do find it a bit tricky!). Please have a look and have a play with the diagrams, the two key thing are:
1. this feature isn't fully automated because EPPI-Reviewer does not assume any pre-established flow, so it needs the user to arrange elements to reflect their own decisions.
2. Thus, users have to select codes from the codetrees, insert them into the diagram explicitly, ask the system to "add code counts" and then drag them around so to produce a neatly organised representation of flows.
I understand this is a little vague, so please do feel free to come back to us with more specific questions, should you need more guidance.