Color and Shape Legends Not Combined in Line Chart or Multiple Charts

Code to reproduce:

In line chart:

df = pd.DataFrame(
    data=[[0, 10, "a"], [1, 20, "a"], [0, 15, "b"], [1, 25, "b"]],
    columns=["x", "y", "c"]
)
alt.Chart(df).mark_line().encode(
    x="x",
    y="y",
    color="c",
    shape="c"
)

Multi Charts:

df = pd.DataFrame(data=[[0, 10, "a"], [1, 20, "a"], [0, 15, "b"], [1, 25, "b"]], columns=["x", "y", "c"])
points = alt.Chart(df).mark_point().encode(x="x", y="y", color="c", shape="c")
lines = alt.Chart(df).mark_line().encode(x="x", y="y", color=alt.Color("c", legend=None))
points + lines

Expected behaviour

The legend should be like this:

[jakevdp]

These are probably questions to raise in the Vega-Lite forums.

One note: the last chart will work as expected if you use a detail encoding as well, which controls how data is grouped without adding a visual attribute:

alt.Chart(source).mark_line().encode(
    x='x:Q',
    y='values:Q',
    color='operation:N',
    shape='shifted:N',
    detail='shifted:N'
)

[joelostblom]

For reference, the corresponding Vega-Lite issue is here https://github.com/vega/vega-lite/issues/5996