So I am playing around with the Cars dataset and am looking to add the R-value to a scatter chart. So I can use this code to produce a scatter chart using transform_regression
to add a regression line which is great.
from vega_datasets import data
import altair as alt
import pandas as pd
import numpy as np
cars = data.cars()
chart = alt.Chart(cars).mark_circle().encode(
alt.X('Miles_per_Gallon', scale=alt.Scale(domain=(5,50))),
y='Weight_in_lbs'
)
chart + chart.transform_regression('Miles_per_Gallon','Weight_in_lbs').mark_line()
Here is the chart
Then I am looking get the R-value. So can use pandas with this code as I am not sure how to get the R-value with Altair.
corl = cars[['Miles_per_Gallon','Weight_in_lbs']].corr().iloc[0,1]
corl
Now I was wondering how would I go about adding the R-value on the chart as a sort of label?
You can do this by adding a text layer:
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"r: {corl:.3f}"),
)
chart + text + chart.transform_regression('Miles_per_Gallon','Weight_in_lbs').mark_line()
In future versions of Altair, the empty data in the chart will no longer be required.