Adding datetime.date Support

[zacharylawrence]

I looked around for an existing issue, but didn't see one. Please let me know if I should respond somewhere else instead!

Currently, putting a datetime.date in an altair chart gives the error:

TypeError: Object of type date is not JSON serializable

Are there ideas to support datetime.date in the future?

For example:

data = pd.DataFrame({
    "time": [datetime.date(2020, 5, 1), datetime.date(2020, 5, 3)],
    "value": [3, 5],
})

alt.Chart(data).mark_line().encode(
    x="time",
    y="value",
)

[jakevdp]

The way to support that would be to update sanitize_dataframe to convert these into serializable ISO timestamps: https://github.com/altair-viz/altair/blob/03f37ca8b2701bf8d9e61646cacf8eee2edcad87/altair/utils/core.py#L243

It hasn't been done yet because it hasn't come up very often: generally timestamps in dataframes come in datetime-typed columns, rather than object-typed columns containing Python dates.

An easy workaround is to do that conversion yourself:

df.time = pd.to_datetime(df.time)

[zacharylawrence]

Thanks for the clear explanation! Would you be open to a PR to this effect?

To add some background, this has come up from reading a parquet file with DATE type column: https://github.com/apache/parquet-format/blob/master/LogicalTypes.md#date

[jakevdp]

Sure, a PR would be great!