Altair Color binned values

Question

Having trouble coloring binned values in the following histogram. I intend on coloring all bars that are less-than 50 on the x-axis (Creditworthiness). How is this done in Altair?

base = alt.Chart(X_train)

histogram = base.mark_bar().encode(
    alt.X('Creditworthiness', bin=True),
    y='count()',
    color=alt.condition(
        alt.datum.Creditworthiness < 50,
        alt.value("steelblue"),  # The positive color
        alt.value("orange")  # The negative color
    )
)

threshold_line = pd.DataFrame([{"threshold": max_profit_threshold}])
mark = alt.Chart(threshold_line).mark_rule(color="#e45755").encode(
    x='threshold:Q',
    size=alt.value(2)
)

histogram + mark

Answer 1

There are two ways to do this; the quick way which is undocumented and may not work in the future, and the more robust way that is a bit more code.

The quick way relies on using the internal field names generated by vega for binned encodings:

import altair as alt
import pandas as pd
import numpy as np

np.random.seed(1701)
X_train = pd.DataFrame({
    'Creditworthiness': np.clip(50 + 20 * np.random.randn(300), 0, 100)
})

alt.Chart(X_train).mark_bar().encode(
    alt.X('Creditworthiness', bin=True),
    y='count()',
    color=alt.condition(
        alt.datum.bin_maxbins_10_Creditworthiness_end <= 50,
        alt.value("steelblue"),  # The positive color
        alt.value("orange")  # The negative color
    )
)

The documented way is to move your binning from the encoding to an explicit transform, which is a bit more verbose:

alt.Chart(X_train).transform_bin(
    'Creditworthiness_bin', 'Creditworthiness', bin=alt.Bin(step=10)
).transform_joinaggregate(
    count='count()', groupby=['Creditworthiness_bin']  
).mark_bar(orient='vertical').encode(
    alt.X('Creditworthiness_bin:Q', bin='binned'),
    alt.X2('Creditworthiness_bin_end'),
    alt.Y('count:Q'),
    color=alt.condition(
        alt.datum.Creditworthiness_bin_end <= 50,
        alt.value("steelblue"),  # The positive color
        alt.value("orange")  # The negative color
    )
)