alt.Y and alt.datum

[kamilm]

I am trying to learn the library and playing around, I thought I had a decent mental model, but I was surprised that this DID NOT work: using alt.Y(alt.datum(300) throws an error, while y=alt.datum(300) works just fine. Why is that? How should I be thinking about alt.Y, I like the method style rather than the attribute one, but are they not on parity?

Minimal Example:

# add a rule line to a line plot

import altair as alt
from vega_datasets import data


source = data.stocks()

base = alt.Chart(source)
lines = (
    base
    .mark_line()
    .encode(
        alt.X("date:T"),
        alt.Y("price:Q"),
        alt.Color("symbol:N"),
    )
)
rule = (
    base
    .mark_rule(strokeDash=[5,5])
    .encode(
        # this works
        y=alt.datum(300)
        # this does not work
        # alt.Y(alt.datum(300))
    )
)

lines + rule

[kamilm]

I believe what I was looking for is alt.XDatum and alt.YDatum :)