[Chart Plugin] Multi-tier grouping by date displays epoch time in axis label

[brianblakely]

Bug Report

Steps to Reproduce:

1. Go to https://perspective.finos.org/blocks/editable/index.html
2. Switch to X Bar plugin
3. Group by: Ship Date > Product Name

Expected Result:

Date labels rendered on Y axis should match formatting one would see in Datagrid plugin

Actual Result:

Dates appear to be rendered in epoch time

Environment:

Perspective 2.9.0 Chrome 122