Altair transfrom_density

Question

I am trying to replicate this graph in Altair but for some reason is don't get any information on my chart, I can only see an empty chart.

link to data

My attempt:

alt.Chart(data).transform_density('age', as_=['age', 'density'],
).mark_area(color='survived').encode(
    x="age",
    y='density:Q',
)

Answer 1

color needs to be specified as an encoding when grouping by a variable, not as a mark parameter. In this case you also need to group the density calculation as in https://altair-viz.github.io/gallery/density_stack.html

alt.Chart(data).transform_density(
    'age',
    as_=['age', 'density'],
    groupby=['survived']
).mark_area().encode(
    x="age",
    y='density:Q',
    color='survived:N'
)