Add custom legend when using alt.when to define colors used

[laguill]

Hello,
I am new to Altair and would like to display a legend that explains the colors used based on conditions in my chart. Here is an example from the documentation:

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

rand = np.random.RandomState(42)
df = pd.DataFrame({
    'xval': range(100),
    'yval': rand.randn(100).cumsum()
})

slider = alt.binding_range(min=0, max=100, step=1, name='Cutoff ')
selector = alt.param(name='SelectorName', value=50, bind=slider)
predicate = alt.datum.xval < selector

alt.Chart(df).mark_point().encode(
   x='xval',
   y='yval',
   color=alt.condition(predicate, alt.value("red"), alt.value("blue")),
).add_params(
   selector
)

https://altair-viz.github.io/user_guide/interactions/bindings_widgets.html#data-driven-comparisons

How can I modify this code to include a legend that explains what the red and blue colors represent? For example, I want the legend to show "red" for values below the cutoff and "blue" for values above the cutoff.

Thank you!