Altair - Gradient above line - altair

Question

I want to create an area chart, however the gradient should run from the line up to the top of the chart. Any ideas?
example of a regular gradient chart here https://altair-viz.github.io/gallery/area_chart_gradient.html
alt.Chart(source).transform_filter(
'datum.symbol==="GOOG"'
).mark_area(
line={'color':'darkgreen'},
color=alt.Gradient(
gradient='linear',
stops=[alt.GradientStop(color='white', offset=0),
alt.GradientStop(color='darkgreen', offset=1)],
x1=1,
x2=1,
y1=1,
y2=0
)
).encode(
alt.X('date:T'),
alt.Y('price:Q')
)

Answer 1

You can do this by setting the y2 encoding to alt.value(0) – the zero in this case measures pixels from the top of the chart axis:
import altair as alt
from vega_datasets import data
source = data.stocks()
alt.Chart(source).transform_filter(
'datum.symbol==="GOOG"'
).mark_area(
line={'color':'darkgreen'},
color=alt.Gradient(
gradient='linear',
stops=[alt.GradientStop(color='white', offset=0),
alt.GradientStop(color='darkgreen', offset=1)],
x1=1,
x2=1,
y1=1,
y2=0
)
).encode(
alt.X('date:T'),
alt.Y('price:Q'),
y2=alt.value(0)
)