python - altair: Access rSquared-value in a regression

Question

I am using this example https://altair-viz.github.io/user_guide/transform/regression.html for plotting a regression trendline in altair.

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

np.random.seed(42)
x = np.linspace(0, 10)
y = x - 5 + np.random.randn(len(x))

df = pd.DataFrame({'x': x, 'y': y})

chart = alt.Chart(df).mark_point().encode(
    x='x',
    y='y'
)

chart + chart.transform_regression('x', 'y').mark_line()

Additionally, I want to add the rSquared-value as text to the chart. How can I access the value? According to the documentation, it should be something like:

chart + chart.transform_regression('x', 'y', params=True).mark_text()

Answer 1

When using mark_text() you'll need to specify the x and y location (or encoding) along with the label of the text value you want to show:

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

np.random.seed(42)
x = np.linspace(0, 10)
y = x - 5 + np.random.randn(len(x))

df = pd.DataFrame({'x': x, 'y': y})

chart = alt.Chart(df).mark_point().encode(
    x='x',
    y='y'
)
line = chart.transform_regression('x', 'y').mark_line()

params = alt.Chart(df).transform_regression(
    'x', 'y', params=True
).mark_text(align='left').encode(
    x=alt.value(20),  # pixels from left
    y=alt.value(20),  # pixels from top
    text='rSquared:N'
)

chart + line + params