Getting this error when the graph has no edges. Still results in a plot, but it's a bit weird.

[ryanpeach]

/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.

[Zsailer]

You're right. We should probably catch+hide this warning. Some people might be interested in drawing a network with no edges.