如何在Altair，Python中使聚合点大小更大？

import altair as alt
import pandas as pd

data = pd.DataFrame({'x': [1, 2, 3, 4, 5],
                     'y': [2, 4, 6, 8, 10],
                     'agg': ['A', 'A', 'B', 'B', 'B']})

chart = alt.Chart(data).mark_circle().encode(
    x='x',
    y='y',
    size=alt.value(100),
    color='agg'
)

chart.show()

import matplotlib.pyplot as plt

x = [1, 2, 3, 4, 5]
y = [2, 4, 6, 8, 10]
agg = ['A', 'A', 'B', 'B', 'B']

plt.scatter(x, y, s=100, c=agg)
plt.show()