[Python] Altair chart empty by dropdown when other chart is displayed

I want to have two different charts, where in the first I can choose an artist in a legend and in the second chart I want to have a dropdown, that filters a year. In my database I have artists, sales and years (for published albums). The charts work fine by itself, but if I want to show both with a vertical concatenation I get a problem. The first chart works as expected but the second graph is completely empty on selecting a year.

import altair as alt
import pandas as pd

# Sample Data
top = pd.DataFrame({
'Artist': ['Artist1', 'Artist2', 'Artist3', 'Artist1', 'Artist2', 'Artist3'],
'Sales': [100, 200, 300, 150, 250, 350],
'Year': ['2020', '2020', '2020', '2021', '2021', '2021'],
})

# Selection for the legend
select_artist_legend = alt.selection_point(fields=["Artist"], bind="legend")

# First plot (cumulative sales over years)
line = alt.Chart(top).transform_window(
cumulative_sales='sum(Sales)',
sort=[{'field': 'Year'}],
groupby=['Artist']
).mark_line().encode(
alt.X("Year:T"),
alt.Y("cumulative_sales:Q"),
alt.Color("Artist:N"),
opacity=alt.condition(select_artist_legend, alt.value(1.0), alt.value(0.1))
).add_params(select_artist_legend)

# Dropdown selection for the year
year_dd = alt.binding_select(options=sorted(top.Year.unique()), name="Year")
year_select = alt.selection_point(fields=["Year"], bind=year_dd, name='year_select')

# Third plot (sales by artist for selected year)
bar = alt.Chart(top).mark_bar().encode(
alt.X("Artist:N"),
alt.Y("Sales:Q"),
alt.Color("Artist:N")
).add_params(year_select).transform_filter(year_select)

# Combine the charts
combined_chart = line & bar
combined_chart


Note: I'm using VS Code and Python Notebooks

I tried debugging and trying to separate the two charts more, didn't seem to help. I also tried stacking bar twice instead of stacking line and bar, but there everything works, so stacking by itself didn't seem to be the problem.

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