How to change decimal format inside a transform_calculate function (Altair)

Question

I'm trying to limit the numbers after the decimal in the text displayed in my graph, but I'm having a hard time doing so inside a transform_calculate function, this is what it looks like : (I am printing the equation to a linear regression under the form y = ax + b))

    params = alt.Chart(df).transform_regression(
        axe_x ,axe_y, params=True
    ).transform_calculate(
        intercept='datum.coef[0]',
        slope='datum.coef[1]'
    ).transform_calculate(
      text= "y = " + alt.datum.intercept + "x + " + alt.datum.slope

    ).mark_text(
        baseline="top",
        align="left"
    ).encode(
        x=alt.value(20),  # pixels from left
        y=alt.value(20),  # pixels from top
        text='text:N',
  )

I've tried many things like

      text= "y = " + f'format(datum.{intercept},".2f")' + "x + " + alt.datum.slope

and

      text= "y = " + alt.Text("alt.datum.intercept", format=".2f)  + "x + " + alt.datum.slope

Current output :

I'm a bit stuck here, help much appreciated thank you

Answer 1

You can use the Vega expression function round to round a value to the nearest integer:

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
).transform_calculate(
    intercept='round(datum.coef[0] * 100) / 100',
    slope='round(datum.coef[1] * 100) / 100',
    text= '["y = " + datum.intercept, "x + " + datum.slope]'
).mark_text(align='left').encode(
    x=alt.value(20),  # pixels from left
    y=alt.value(20),  # pixels from top
    text='text:N'
)

chart + line + params