/usr/local/Cellar/python3/3.6.4_2/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/altair/utils/core.py:81: UserWarning: I don't know how to infer vegalite type from 'empty'. Defaulting to nominal.
Maybe we should test as to whether or not there are edges or nodes in the graph before making charts.
Maybe we should test as to whether or not there are edges or nodes in the graph before making charts.