Chart visual projections structure — Category/Y/Series serialized as a dict under projections.

The chart's projections use a dict where the schema expects arrays of field references per bucket.

Why it happens

• Category, Y or Series serialized as a dict instead of an array.
• A projections shape copied from the wrong visual type.

How to fix it

1. Serialize each bucket (Category, Y, Series) as an array of field-reference objects.
2. Copy the projections block from a proven chart visual.json of the same type.