如何在altair.layered绘图中配置条形图？

import altair as alt
from vega_datasets import data

source = data.barley()

base = alt.Chart(source)

bar = base.mark_bar().encode(
    x='variety',
    y='sum(yield)'
)

text = base.mark_text(
    align='center',
    baseline='bottom',
    dy=-5
).encode(
    x='variety',
    y='sum(yield)',
    text='sum(yield)'
)

chart = bar + text

chart = chart.properties(
    title='Bar Chart Example',
    width=400,
    height=300
)

chart = chart.interactive()

chart.show()