This might be similar to Color in vega bar charts but I'm not 100% sure (in that case you just have to specify scale: null.
To make it easier to follow, could you export the spec with data inlined by
- for 7.10: Going to "Inspect" in the top right, switch "Requests" to "Vega debug", select the "spec" tab and copy the result
- for older versions: follow these instructions: https://gist.github.com/nyurik/736c34970ba3c32f3fe3d45d66719b3e