I think that this is an excellent idea.
A diagram like this very clearly and quickly defines the scope of the site, in fact I think simple diagrams like this would be worthwhile having in every stack exchange site FAQ.
I do have a few suggestions though:
- The red circle representing All people who use computers isn't really necessary.
It might be useful in the context of another FAQ question on Are IT related question on topic here? but in the main What kind of questions can I ask here? FAQ entry it just adds unnecessary confusion.
I think the original Question applies to actually works better than On topic if question applies to, since the circles are either on topic or off topic, depending on whether they are filled or empty.
- Replace Professional setting with All workplaces
This makes it consistent with the other sets and means that when you put it together with the diagram title the sentence scans. I.e. the on-topic circle would read Question applies to all workplaces rather than On topic if question applies to professional setting.