如何在Altair中绘制带有中线的预分格直方图？

import altair as alt
from vega_datasets import data

source = data.cars()

chart = alt.Chart(source)

x = alt.X('Miles_per_Gallon', bin=True)
y = alt.Y('count()', axis=alt.Axis(title='Frequency'))

histogram = chart.mark_bar().encode(
    x=x,
    y=y
)

rule = chart.mark_rule(color='red').encode(
    x='mean(Miles_per_Gallon)'
)

chart = (histogram + rule).properties(
    width=600,
    height=400
)

chart.show()