Timezone information dropped from polars dataframe #3921
Description
What happened?
When plotting from a polars dataframe that has a datetime with timezone the timezone is not passed on to vega-lite. This is a noticable problem when you are plotting data across a time change. Simple example below:
import polars as pl
import altair as alt
from polars import col as c
start_datetime = pl.datetime(2023, 11, 5, time_zone='US/Mountain')
datetimes = pl.datetime_range(
start_datetime,
start_datetime.dt.offset_by("3h"),
"1h",
closed="both",
eager=True
)
# Create a Polars DataFrame with a datetime column (with timezone)
df = pl.DataFrame({
'datetime': datetimes,
'value': [10, 20, 30, 40]
})
# Plot with Altair using the datetime column (with timezone)
color = alt.Color("value:N", title="Value")
chart_dt = alt.Chart(df).mark_point().encode(
alt.X('datetime:T', title='Datetime (with timezone)'),
alt.Y('value:Q', title='Value'),
color
).properties(title='Altair plot: Datetime with timezone')
Looking at the data supplied to vega-lite in the editor, the timezone information has been dropped, so vega-lite does not know that the second 1 AM data point is at the later 1 AM.
"datasets": {
"zzz": [
{"datetime": "2023-11-05T00:00:00", "value": 10},
{"datetime": "2023-11-05T01:00:00", "value": 20},
{"datetime": "2023-11-05T01:00:00", "value": 30},
{"datetime": "2023-11-05T02:00:00", "value": 40}
]
}If the datetime column is manually converted to a string the chart appears as expected:
# Plot with Altair using the string column
chart_str = alt.Chart(df.with_columns(c.datetime.dt.to_string())).mark_point(point=True).encode(
alt.X('datetime:T', title='Datetime (as string)'),
alt.Y('value:Q', title='Value'),
color
).properties(title='Altair plot: Datetime as string')
chart_str
"datasets": {
"zzz": [
{"datetime": "2023-11-05 00:00:00.000000-06:00", "value": 10},
{"datetime": "2023-11-05 01:00:00.000000-06:00", "value": 20},
{"datetime": "2023-11-05 01:00:00.000000-07:00", "value": 30},
{"datetime": "2023-11-05 02:00:00.000000-07:00", "value": 40}
]
}A molab replication of this is here
What would you like to happen instead?
Altair should preserve timezone information if it exists in the dataframe and the example plot should display as it does when converting to string manually.
Which version of Altair are you using?
6.0.0