Dynamically Sorting Facets in Altair Based on Different Metrics

[ggallitz]

Hi everyone,
I’m working on an Altair facet chart where each facet represents a country, and inside each facet, I display three key metrics:

• Military spending (spend)
• Population (pop)
• GDP (bip)
  I want to allow users to dynamically sort the country facets based on one of these three metrics via a dropdown menu. However, while the sorting parameter updates correctly, the facet order does not change dynamically as expected.
  Here’s my code:

`import altair as alt
import pandas as pd

Sample Data: Military spending, population, and GDP per country

df = pd.DataFrame({
"country": ["USA", "China", "Russia", "India", "Germany", "UK"],
"spend": [801, 293, 65, 76, 56, 68], # Military Spending (Billion USD)
"pop": [331, 1441, 143, 1393, 83, 67], # Population (Million people)
"bip": [23, 17, 1.5, 3, 4.5, 3.1], # GDP (Trillion USD)
})

Convert data from wide to long format for Altair

df_melted = df.melt(id_vars=["country"], var_name="metric", value_name="value")

Sorting dropdown for facets

facet_sort_dropdown = alt.binding_select(
options=["spend", "pop", "bip"], name="Sort Countries by: "
)
facet_sort_param = alt.param(value="spend", bind=facet_sort_dropdown)

Chart with dynamically sorted facets

chart = alt.Chart(df_melted).mark_bar().encode(
x="value:Q",
y=alt.Y("metric:N", title=""), # Show Spend, Population, and GDP as categories
color="metric:N" # Different colors for each metric
).facet(
row=alt.Row(
"country:N",
sort=alt.EncodingSortField(field="sort_order", order="descending") # Sort dynamically
)
).add_params(facet_sort_param
).transform_lookup(
lookup="country", from_=alt.LookupData(df, "country", ["spend", "pop", "bip"]) # Assign values for sorting
).transform_calculate(
sort_order=f"datum[{facet_sort_param.name}]" # Apply dynamic sorting
)

chart.show()`

The dropdown updates the sorting parameter, but the facet order remains unchanged.
I expected the facets (countries) to reorder dynamically based on the selected metric (spend, pop, bip), but it seems like Altair does not apply dynamic sorting to facets.
Question:
Is there a way to make Altair update the facet order dynamically when selecting a different sorting metric?
If Altair does not support this natively, is there a workaround to achieve dynamic facet sorting?

Any insights would be much appreciated! Thanks in advance.
Best,
Georg