How to add custom spacing in columns of bar charts?

[moolieloo]

import altair as alt
from vega_datasets import data

source = data.barley()

alt.Chart(source).mark_bar().encode(
    x='year:O',
    y='sum(yield):Q',
    color='year:N',
    column='site:N'
)

Question

I'd like to add custom spacing between the columns. For example, Crookston and Duluth (first two cols) should be closer together than Grand Rapids, Morris and University Farm. And Waseca should be more spaced out from University Farm. Is there a way to do this in Altair?