Adding vertical average lines on top of a layered histogram in Altair - altair

Question

I am trying to add vertical lines indicating the average of datasets in a layered histogram in Altair (based on their example). My attempt below is failing:
base = alt.Chart(outcomes)
bar = base.transform_fold(
['Push','Dealer Win','Player Win','Ace High Push'],
as_=['Outcome','Outcomes out of 1000']
).mark_bar(
opacity=0.3,
binSpacing=0
).encode(
alt.X('Outcomes out of 1000:Q', bin=alt.Bin(maxbins=100)),
alt.Y('count()', stack=None),
alt.Color('Outcome:N')
)
rule = base.transform_fold(
['Push','Dealer Win','Player Win','Ace High Push'],
as_=['Count','Outcome']
).mark_rule(
color='red'
).encode(
alt.X('mean(Outcome):Q'),
size=alt.value(2)
)
bar + rule
which results in:
When I do just bar though the layered histogram renders just fine:
Basically what I'm looking for is:
Thanks🙏
Update (less than an hour after original post):
Thanks #debbes for the speedy guidance! I was able to use your example to get this working via:
base = alt.Chart(outcomes).transform_fold(
['Push','Dealer Win','Player Win','Ace High Push'],
as_=['Outcome','Outcomes out of 1000']
).transform_bin(
field='Outcomes out of 1000',
as_='bin_meas',
bin=alt.Bin(maxbins=100)
).encode(
color='Outcome:N'
)
hist = base.mark_bar(
opacity=0.3,
binSpacing=0
).encode(
alt.X('bin_meas:Q'),
alt.Y('count()', stack=None)
)
rule = base.mark_rule(
size=2
).encode(
alt.X('mean(Outcomes out of 1000):Q')
)
hist + rule
which results in:

Answer 1

In this case you have to use the transform_bin instead of doing the binning in the X encoding:
base = alt.Chart(source).transform_fold(
['Trial A', 'Trial B', 'Trial C'],
as_=['Experiment', 'Measurement']
).transform_bin(
field='Measurement',
as_='bin_meas',
bin=alt.Bin(maxbins=100)
).encode(
color='Experiment:N'
)
hist = base.mark_bar(opacity=0.3,binSpacing=0).encode(
alt.X('bin_meas:Q'),
alt.Y('count()', stack=None),
)
rule = base.mark_rule(size=2).encode(alt.X('mean(Measurement):Q'),)
hist + rule