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#you can use the labelExpr that is inside the alt.Axis object
alt.Chart(df).mark_bar().encode(
x=alt.X('Variable', bin=True, axis=alt.Axis(labelExpr="datum.label == 0 ? 1 : datum.label")),
y='Variable2',
color=alt.value('some_color')
)
# like you see labelExpr works like a kind of conditional statement
#where if the datum label if this change it for this other thing