docs: Fix "Layered chart with Dual-Axis" (Method syntax)

[dangotbanned]

Discovered while writing tests for #3659.

The User Guide displays the attribute syntax, but the method syntax one was not equivalent.

Results in no titleColor being applied and is silently ignored during validation.

Previous

Note: the warning from pylance is only showing due to some local changes I have on-top of (1a39019)

Code block

import altair as alt
from vega_datasets import data

source = data.seattle_weather()

base = alt.Chart(source).encode(alt.X("month(date):T").title(None))

area = base.mark_area(opacity=0.3, color="#57A44C").encode(
    alt.Y("average(temp_max)").title("Avg. Temperature (°C)", titleColor="#57A44C"),
    alt.Y2("average(temp_min)"),
)

line = base.mark_line(stroke="#5276A7", interpolate="monotone").encode(
    alt.Y("average(precipitation)").title(
        "Precipitation (inches)", titleColor="#5276A7"
    )
)

alt.layer(area, line).resolve_scale(y="independent")

Fixed

Code block

import altair as alt
from vega_datasets import data

source = data.seattle_weather()

base = alt.Chart(source).encode(alt.X("month(date):T").title(None))
area = base.mark_area(opacity=0.3, color="#57A44C").encode(
    alt.Y("average(temp_max)").axis(
        title="Avg. Temperature (°C)", titleColor="#57A44C"
    ),
    alt.Y2("average(temp_min)"),
)

line = base.mark_line(stroke="#5276A7", interpolate="monotone").encode(
    alt.Y("average(precipitation)").axis(
        title="Precipitation (inches)", titleColor="#5276A7"
    )
)

alt.layer(area, line).resolve_scale(y="independent")