Add diagram

[erica-chiu]

Resolves #113

[csala]

Hey @erica-chiu ! Thanks for the PR. It seems like travis is failing because it is missing the graphviz dependency when running the tests.

Would you mind moving the 'graphviz>=0.9', specification in setup.py (line 59) from development_requires (you can remove it from here) to install_requires?
Also, while you are at it, can you add an upper maximum and make it 'graphviz>=0.9,<1',?

It should look like this:

install_requires = [
    'graphviz>=0.9,<1',
]