如何在从Altair提取的rSquared中添加前缀? [英] How to add Prefix to rSquared extracted from Altair?

问题描述

我正在使用

I'm adding the rSquared to a chart using the method outlined in this answer:

r2 = alt.Chart(df).transform_regression('x', 'y', params=True
).mark_text().encode(x=alt.value(20), y=alt.value(20), text=alt.Text('rSquared:N', format='.4f'))

But I want to prepend "rSquared = " to the final text.

I've seen this answer involving an f string and a value calculated outside the chart, but I'm not clever enough to figure out how to apply that solution to this scenario.

I've tried, e.g., format='rSquared = .4f', but adding any additional text breaks the output, which I'm sure is the system working as intended.

解决方案

One possible solution using the posts you linked to would be to extract the value of the parameter using altair_transform and then add the value to the plot. This is not the most elegant solution but should achieve what you want.

# pip install git+https://github.com/altair-viz/altair-transform.git

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


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  = chart.transform_regression('x','y', params=True).mark_line()
R2 = altair_transform.extract_data(params)['rSquared'][0]


text = alt.Chart({'values':[{}]}).mark_text(
    align="left", baseline="top"
).encode(
    x=alt.value(5),  # pixels from left
    y=alt.value(5),  # pixels from top
    text=alt.value(f"rSquared = {R2:.4f}"),
)


chart + line + text

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