Vega 2.x does not support non-array data objects, so it is not possible to access license and sources fields from the structured data inside the graph itself.
As a workaround, until Vega 3.x is available, non-array fields will need to be wrapped into a single value array, e.g.
"meta": [{ "description": "...", "license_code": "...", "license_text": "...", "license_url": "...", "sources": "...", // These are for maps only: "zoom": "...", "latitude": "...", "longitude": "..." }]