I have to admit, diagrams are a bit harder to do in text, especially interconnected graphs. That said, I could imagine something that uses triples to define a rendered graph, similar to how RDF for the semantic web works. Say

     #BEGIN_GRAPH
     'Paul Graham' founded YC 
     'Sam Altman' leads YC 
     YC funded AirBnB
     YC funded Reddit
     AirBnB <> Accommodation
     #END_GRAPH

Leads to a graph like

      [Paul Graham]----founded----->[YC]------funded----->[AirBnB]<------>[Accommodation]
      [Sam Altman]-----leads------->[  ]            `---->[Reddit]
      

Now that could be prettier, but I am too lazy to copy and paste box drawing characters. With a simplified form like that, all the data is in the form of subject predicate object sentences, which makes for easy parsing. The data could also be fed into a diagram making application like yED for pretty graphical output.