[Python] altair mark_area() with unexpected behaviour of y2

I'm trying to make an area chart of a CDF plot (which should be always increasing), instead I get the result in the image below.

import altair as alt
import polars as pl

_data = pl.DataFrame({"outcomes": [16950, 17050, 18750, 18750, 20950]})
(
alt.Chart(
_data,
)
.transform_quantile("outcomes", step=0.1)
.mark_area(line=True, opacity=0.5)
.encode(
alt.X("value:Q"),
alt.Y("prob:Q").title("Prob"),
# alt.Y2(alt.datum(0))
# y2=alt.datum(0),
)
)


(I can't post images because I have <10 reputation, so I leave a link instead)

error1

If I uncomment the alt.Y2(...) line I still don't get what I'm looking for (namely, there's no area being plotted, although it's at least the actual CDF).

error2

When I use the last formulation y2=alt.datum(0) it does work.

Questions

1. I expected Y2 to be 0 by default (I recall having read it somewhere but I cannot find it not, maybe I made it up?). It does seem to be so most of the time, when is it not? Why?
2. I expected the alt.Y2(...) API to behave exactly the same as y2=... but it does not seem to be the case (I've had problems with this in other plots involving different channels too). When are they different? Why? Or am I doing something wrong?

Regards.

Versions

• altair==5.5.0
• polars==1.30.0

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