Shared axis labels with independent scale - altair

Question

When facet/concat-ing charts, I would like the axis labels to be shared (so only 1 label per column/row, here: Horsepower), but the scale to be independent. Is this possible?
I thought a combination of resolve_axis and resolve_scale would be the way to go, as the title is a part of Axis, but I didn't get it to work.
I'm also wondering what resolve_axis actually does different than resolve_scale, anyone has an example?
base = alt.Chart(source).mark_circle().encode(
x=alt.X('Horsepower:Q',),
y=alt.Y('Miles_per_Gallon:Q'),
color='Origin:N',
row=alt.Row('Origin:N'),
).properties(
width=200, height=100
)
base.resolve_axis(
x='shared' # doesn't do anything obvious
).resolve_scale(
x='independent'
)
Open the Chart in the Vega Editor

Answer 1

I found a hacky way to do this, by misusing the facet header:
base = alt.Chart(source).mark_circle(size=60).encode(
x=alt.X('Horsepower:Q',),
y=alt.Y('Miles_per_Gallon:Q',
axis=alt.Axis(title=''),),
color='Origin:N',
column=alt.Column('Origin:N', header=alt.Header(title='Miles_per_Gallon')),
).properties(
width=200, height=200
).configure_header(
labelExpr="['Origin',datum.value]",
titleOrient='left'
)
display(base.resolve_scale(y='shared'))
display(base.resolve_scale(y='independent'))

Answer 2

I don't know of any way to do what you're hoping for (independent scales with only a single outer axis title) via scale and guide resolution.
As to your question of the difference between resolve_scale and resolve_axis, an example may help.
Here's a chart with independent y scale:
import altair as alt
from vega_datasets import data
source = data.cars()
base = alt.Chart(source).mark_circle().encode(
x=alt.X('Horsepower:Q',),
y=alt.Y('Miles_per_Gallon:Q'),
color='Origin:N',
column=alt.Column('Origin:N'),
).properties(
width=150, height=150
)
base.resolve_scale(
y='independent'
)
And here's one with independent y axis:
base.resolve_axis(
y='independent'
)
In both cases, each chart gets its own axis (because independent scales imply independent axes), but only with an independent scale do the axes scales differ from each other.