Specify one series to be plotted on the secondary y axis

I'm not an expert and I'm sure there are more efficient approaches. I used a filter to layer each graph and added an independent setting for the y-axis.

import altair as alt
from altair import datum
import numpy as np
import pandas as pd

np.random.seed(20200113)
df = (
    pd.DataFrame({
        'a': np.random.rand(10),
        'b': np.random.rand(10),
        'c': np.random.rand(10) * 10,
    })
    .stack()
    .to_frame('value')
    .reset_index()
)
df.columns = ['index', 'key', 'value']

base = alt.Chart(df).mark_line().encode(
    x='index',
    y='value',
    color='key:N'
    ).transform_filter(
    (datum.key == 'a') | (datum.key =='b')
)

line = alt.Chart(df).mark_line(color='#57A44C').encode(
    x='index',
    y=alt.Y('value', axis=alt.Axis(title='value')),
    color='key'
).transform_filter(
    (datum.key == 'c')
)

alt.layer(base, line).resolve_scale(
    y = 'independent'
)