'Open Vegalite in notebook' can't work with local url file path

[zzhangjii]

The context menu command to open a vl.json file in Notebook doesn't work if the vl.json is using local url as its data source:

if the ''data' inside the vl.json looks like this:
"data":{"url":"./data/barley.json","format":{"type":"json"}}

then in notebook
data_src = json.load(json_data)
alt.Chart.from_dict(data_src)
will not display the correct graph, because data reading fails.

It seems currently Altair's from_dict() function only works with inline data or online url