Apparently "object" is not accessible directly by the Vega syntax, only arrays are. Unless there is some trick I don't know about, we will need to introduce a workaround for the tabular data non-array elements to Graphs. So fields like sources, license, and description need to be wrapped in an array object - e.g.

Example:

"meta": [ {
  "sources": ...
  "description": ...
  "license": ...
} ]