Altair: add numbers of observation per group

Question

How can I add the numbers of observations per group to a boxplot with altair?

I am looking for something like this:

But I am also open to other solutions (as long as the information is visible).

For instance, how would I add this information to the example plot from here?

import altair as alt
from vega_datasets import data

source = data.population.url

alt.Chart(source).mark_boxplot(extent='min-max').encode(
    x='age:O',
    y='people:Q'
)

Thank you so much for your help!

Answer 1

You cannot dynamically modify titles in Altair/Vega-Lite, so you would need to add a text mark with this information. Something like this (just a proof of principal, you would likely add some explanatory text and make it prettier):

import altair as alt
from vega_datasets import data

source = data.population.url

boxes = alt.Chart(source).mark_boxplot(extent='min-max').encode(
    x='age:O',
    y='people:Q'
)

boxes + boxes.mark_text().encode(
    y=alt.datum(11_500_000),
    text='count()'
)

Two other options are to make the sizes of the boxes correspond to the count or add it as the tooltip.