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

key_x = "Whatever X Is"
key_col = "Color Parameter"
key_y = "Performance or Error"

np.random.seed(0)

charts = []

for sweep, title in zip([
    [0.0, 0.1, 0.5, 0.95],
    [0.0, 0.1, 0.5, 0.75, 1.0, 10.0],
    [0.01, 0.05, 0.1],
], [
    "Parameter 1",
    "Parameter 2",
    "Parameter 3",
]):
    x = []
    y = []
    col = []

    for col_val in range(3):
        for x_val in sweep:
            for seed in range(100):
                col += [col_val]
                x += [x_val]
                y += [np.random.normal(col_val, x_val)]

    df = pd.DataFrame({
        f"{key_x}": x,
        f"{key_y}": y,
        f"{key_col}": col,
    })

    bars = alt.Chart(df, width=600, height=100).mark_bar(tooltip=True).encode(
        x=alt.X(
            f"{key_x}:N",
            axis=alt.Axis(labelAngle=0, title=title),
        ),
        y=alt.Y(f"mean({key_y}):Q"),
        color=alt.Color(f"{key_col}:N"),
        xOffset=alt.XOffset(f"{key_col}:N"),
    )

    charts.append(bars)

chart = alt.vconcat(*charts)

chart.save("altair_concat.html")

The three charts in concatenate have different number of x values but it seems that the first one dictates the width and the bar width of the next ones. How can I make them independent?