`split_by` with a `"date"` typed column produces empty Datagrid columns

[Andrew Stein]

Bug Report

Steps to Reproduce:

Open the Superstore Perspective example and try this config:

{"version":"2.8.0","plugin":"Datagrid","plugin_config":{"columns":{},"editable":true,"scroll_lock":false},"settings":true,"theme":"Pro Light","title":null,"group_by":["State"],"split_by":["New Column 1"],"columns":["Row ID"],"filter":[],"sort":[],"expressions":{"New Column 1":"bucket(\"Order Date\",'Y')"},"aggregates":{}}

Environment:

2.8.0

Expected Result:

Column headers should show string-formatted dates, and the columns should be populated.

Actual Result:

Column headers are unix timestamps and columns are all blank (Datagrid only).