Vega-Lite/Altair extend regression line to the edges of the graph

Question

I'm trying to find a way to extend regression lines in vega-lite/altair charts to the edge of the chart. As of now, when applying a regression transform to a dataset results in datapoints that only stretch to the bounding-box of the original dataset. Is it possible somehow to extend this range to the x/y extents of the chart? In the picture below, the black line is what vega-lite calculates per default. Extending the line to the edges as shown in yellow is what I'm trying to achieve.

EDIT

When specifying the extent property on the transform_regression call it seems like it is adjusting the y variable instead of the x variable. Maybe I'm grossly misunderstanding something but maybe it has something to do with the fact that my x variable are dates which might behave differently?

When I specify the extent like so

CDR_base.transform_regression(
    'per_capita', 
    'year', 
    groupby=['region'],
    extent=[2000, 2100]
  ).mark_line()

I would expect the extent of the regression lines to extend from 2000 to 2100. For some reason the extent gets applied to the y axis it seems.

Answer 1

You can use the extent argument of the regression transform to control the extent of the line. For example, here is a dataset with a default line:

import altair as alt
import pandas as pd
import numpy as np
np.random.seed(2)
df = pd.DataFrame({
    'x': np.random.randint(0, 100, 10),
    'y': np.random.randint(0, 100, 10)
})
points = alt.Chart(df).mark_point().encode(
    x='x:Q',
    y='y:Q'
)
points + points.transform_regression('x', 'y').mark_line()

And here it is with extent set:

points + points.transform_regression('x', 'y', extent=[0, 90]).mark_line()