Altair: merging legends in layer charts

Question

I have a point chart encoding color, size, and shape. By itself all three encodings are collapsed as a single legend. However layering a line chart that shares the same color results in the duplicated legend below. There is no difference when marking size and shape as independent but color as shared. When I disable the legend in the line chart's color encoding, the [top] color legend and all color information disappears. What can I do?

rand = np.random.RandomState(0)
data = pd.DataFrame\
    ( rand.randint(100,500,(15,2))
    , index=[*["a"]*5,*["b"]*5,*["c"]*5]
    , columns=["x","y"]
    ).rename_axis("k").reset_index()

chart1 =\
    ( alt.Chart(data)
    . mark_point()
    . encode(x="x",y="y",color="k",shape="k",size="k")
    )

chart2 =\
    ( alt.Chart(data)
    . transform_regression
        ( on="x"
        , regression="y"
        , groupby=["k"]
        , method="poly"
        )
    . mark_line()
    . encode(x="x",y="y",color="k")
    )

chart =\
    ( (chart1 + chart2)
    . interactive()
    . properties(width="container")
    )

Answer 1

I don't think there's any good answer for this question. There's an open issue about it here.

One workaround is to hide the line legend and set the others to independent:

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

rand = np.random.RandomState(0)
data = pd.DataFrame\
    ( rand.randint(100,500,(15,2))
    , index=[*["a"]*5,*["b"]*5,*["c"]*5]
    , columns=["x","y"]
    ).rename_axis("k").reset_index()

chart1 =\
    ( alt.Chart(data)
    . mark_point()
    . encode(x="x",y="y",color="k",shape="k",size="k")
    )

chart2 =\
    ( alt.Chart(data)
    . transform_regression
        ( on="x"
        , regression="y"
        , groupby=["k"]
        , method="poly"
        )
    . mark_line()
    . encode(x="x",y="y",color=alt.Color("k", legend=None))
    )

chart =\
    ( (chart1 + chart2)
    . interactive()
    ).resolve_scale(color='independent', shape='independent', size='independent')