Dependency such as graphviz is not explicitly defined in docs

I am not sure whether I have missed this or something but I can't see `graphviz` been defined as core dependency for this project. I figured out whenever I do `mkdocs serve` it failed. And tracing back the error I found that the script in `bin/generate_db_schema.py` fails silently. 
Is there a way we can handle this more explicitly? 