Cannot add regression line to repeated scatter plots in Altair

Question

I cannot seem to add a regression line to my repeated scatter plots in Altair as I do not know what to put for x and y in transform_regression('x', 'y').mark_line(). Here's my code, I used cars from vega_datasets.

from vega_datasets import data


source = data.cars()
source 

scatter3 = alt.Chart(source, width=150, height=150).mark_circle().encode(
    alt.X(alt.repeat("row"), type='quantitative'),
    alt.Y(alt.repeat("column"), type='quantitative'),
    alt.Color('Name', legend=None, scale=alt.Scale(scheme='tableau20'))).repeat(
    row=['Miles_per_Gallon', 'Cylinders', 'Displacement', 'Horsepower', 'Weight_in_lbs', 'Acceleration'],
    column=['Miles_per_Gallon', 'Cylinders', 'Displacement', 'Horsepower', 'Weight_in_lbs', 'Acceleration'])
           
scatter3 + scatter3.transform_regression('row', 'column').mark_line()

Can someone help me with the syntax for the last line?

Answer 1

Ah, I see what you want to do. Unfortunately this is not possible yet in VegaLite, which is what Altair is built upon. You can follow the progress in the following two issues:

• https://github.com/vega/vega-lite/issues/7398
• https://github.com/vega/vega-lite/issues/2518

One way to get around this is to use faceting as shown in the answer here Quick way to visualise multiple columns in Altair with regression lines