Drill-downs in Vega-Altair

[ShafeekaNisthar]

Hey Everyone!

I've been trying to create a drill-down visual using Vega-Altair(at least a single level drill down).
I've tried to do so by layering 2 charts, with one on top of another and making the datapoints transparent and appear on click. This has been somewhat counter-intuitive since for bar-charts, the axis for chart 2 is already visible and if i add a tooltip and hover over an empty space, i can still see the data point details. Moreover, i haven't been able to directly say since "Asia" bar is clicked on, transform and show me the details for countries within "Asia", i can merely associate the click to highlight or isolate a given bar, not necessarily drilling down in that case.

some of the code(Please note that i am still a beginner and may have made a few errors), I've attempted is below;

import altair as alt
from vega_datasets import data

source = data.barley()

click = alt.selection_point(encodings=['x'], name='click')

chart1 = alt.Chart(source).mark_bar().encode(
    x='variety:N',
    y='sum(yield):Q',
    color=alt.condition(click, alt.value('steelblue'), alt.value('transparent')),
    tooltip=['variety:N','sum(yield):Q']
).properties(width=600, height=400)

chart2 = alt.Chart(source).mark_bar().encode(
    x='site:N',
    y='sum(yield):Q',
    tooltip=['site:N','sum(yield):Q'],
    color=alt.condition(click, alt.value('transparent'), alt.value('orange')),
).properties(width=600, height=400)

(chart1 + chart2).add_params(click)

import altair as alt
from vega_datasets import data

source = data.cars.url

click = alt.selection_point()

chart1 = alt.Chart(source).mark_circle().encode(
    x='Horsepower:Q',
    y='Miles_per_Gallon:Q',
    tooltip = ['Miles_per_Gallon:Q', 'Horsepower:Q'],
    color=alt.condition(click, alt.value('steelblue'), alt.value('transparent'))
    ).properties(
        width=600, height=400)

chart2 = alt.Chart(source).mark_circle().encode(
    x='Acceleration:Q',
    y='Miles_per_Gallon:Q',
    tooltip = ['Miles_per_Gallon:Q', 'Acceleration:Q'],
    color=alt.condition(click, alt.value('transparent'), alt.value('orange'))
).properties(width=600, height=400)

(chart1 + chart2).add_params(click)

I would appreciate a response, maybe even a direction to look into.

Thank you & Best Regards