Timezone-aware timestamps are displayed in UTC instead of their source timezone

[kjgoodrick]

Timezone-aware timestamps are displayed in UTC

Thank you all for your work bringing Mosaic to Python! I have been testing moving some of my Altair workflows to Mosaic and have been very impressed with the results. I wanted to share one bug I have noticed so far. I'm guessing the real fix for this will be D3 adopting the Temporal API (maybe, someday), but the Vega-Lite behavior of showing time in the browser's timezone seems like a better tradeoff until then (though my preference would certainly be for it to show in the given timezone if doing so is reasonable).

Summary

When a timezone-aware Polars Datetime column is passed to Mosaic through
vgplot, the source timezone appears to be ignored when rendering the
temporal axis.

For example, midnight in America/Denver is displayed as 07:00 rather than 00:00.

Minimal reproduction

from datetime import datetime
from zoneinfo import ZoneInfo

import polars as pl
import vgplot as vg

mountain = ZoneInfo("America/Denver")

df = pl.DataFrame(
    {
        "datetime": [
            datetime(2029, 1, 1, hour, tzinfo=mountain)
            for hour in range(0, 24, 6)
        ],
        "value": [0, 1, 2, 3],
    }
)

print(df.schema)
print(df["datetime"][0])
# Datetime(time_unit='us', time_zone='America/Denver')
# 2029-01-01 00:00:00-07:00

view = vg.plot(
    vg.line_y(
        vg.source("data"),
        x="datetime",
        y="value",
    ),
    data={"data": df},
)

view.show(data={"data": df})

Expected behavior

The first data point should appear at midnight, and the x-axis should show
Mountain Time:

2029-01-01 00:00
2029-01-01 06:00
2029-01-01 12:00
2029-01-01 18:00

Actual behavior

The timestamps appear to be rendered in UTC. The first point is shown at
07:00 instead of 00:00.

Workaround

I can sort of workaround this by stripping away the timezone data, but this creates problems around the DST boundary.

Screenshot

Environment

• mosaic-widget==0.23.0
• vgplot==0.31.0
• Python 3.12