Horizon Chart — Altair

A horizon chart displays many time series compactly by folding values into color-coded bands, preserving local resolution while minimizing vertical space. It divides the y-axis into bands and uses color intensity to encode magnitude, allowing dozens of series to be compared in limited space. This technique is particularly effective when monitoring many metrics simultaneously where traditional line charts would become unreadable.

Horizon Chart rendered with Altair

Python source (Altair)

""" anyplot.ai
horizon-basic: Horizon Chart
Library: altair 6.1.0 | Python 3.13.13
Quality: 78/100 | Updated: 2026-05-07
"""

import os

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


THEME = os.getenv("ANYPLOT_THEME", "light")
PAGE_BG = "#FAF8F1" if THEME == "light" else "#1A1A17"
ELEVATED_BG = "#FFFDF6" if THEME == "light" else "#242420"
INK = "#1A1A17" if THEME == "light" else "#F0EFE8"
INK_SOFT = "#4A4A44" if THEME == "light" else "#B8B7B0"

# Data: Generate server metrics over 24 hours for multiple servers
np.random.seed(42)

n_points = 96
hours = pd.date_range("2024-01-15 00:00", periods=n_points, freq="15min")

data_list = []
servers = ["Web Server 1", "Web Server 2", "Database", "Cache", "API Gateway", "Worker"]

for server in servers:
    t = np.linspace(0, 2 * np.pi, n_points)
    if server == "Database":
        base = 40 + 30 * np.sin(t - np.pi / 2) + np.random.randn(n_points) * 8
    elif server == "Cache":
        base = 25 + np.random.randn(n_points) * 5
        base[40:45] += 40
    elif server == "Worker":
        base = 20 + 50 * (np.sin(t * 3) > 0.7) + np.random.randn(n_points) * 6
    else:
        base = 30 + 25 * np.sin(t - np.pi / 3) + np.random.randn(n_points) * 10

    values = base - base.mean()

    for hour, val in zip(hours, values, strict=True):
        data_list.append({"date": hour, "value": val, "series": server})

df = pd.DataFrame(data_list)

# Horizon chart parameters
n_bands = 3
band_height = df["value"].abs().max() / n_bands

# Create band data with human-readable labels
band_data = []
intensity_labels = {0: "Low", 1: "Medium", 2: "High"}

for _, row in df.iterrows():
    val = row["value"]
    for band in range(n_bands):
        band_min = band * band_height
        if val >= 0:
            band_val = max(0, min(val - band_min, band_height))
            band_data.append(
                {
                    "date": row["date"],
                    "series": row["series"],
                    "band": band,
                    "value": band_val,
                    "direction": "positive",
                    "label": f"Positive {intensity_labels[band]}",
                }
            )
        else:
            abs_val = abs(val)
            band_val = max(0, min(abs_val - band_min, band_height))
            band_data.append(
                {
                    "date": row["date"],
                    "series": row["series"],
                    "band": band,
                    "value": band_val,
                    "direction": "negative",
                    "label": f"Negative {intensity_labels[band]}",
                }
            )

band_df = pd.DataFrame(band_data)

# Colorblind-safe diverging colors using BrBG-inspired palette
# Positive: shades of blue-green, Negative: shades of brown/red
positive_colors = ["#d4e9f0", "#5ba3c0", "#1d6a8e"]
negative_colors = ["#f0d4a8", "#d4944f", "#8b5a2b"]

band_df["color_label"] = band_df["label"]

color_scale = alt.Scale(
    domain=["Positive Low", "Positive Medium", "Positive High", "Negative Low", "Negative Medium", "Negative High"],
    range=positive_colors + negative_colors,
)

# Create the horizon chart
chart = (
    alt.Chart(band_df)
    .mark_area()
    .encode(
        x=alt.X("date:T", title="Time", axis=alt.Axis(format="%H:%M", labelFontSize=18, titleFontSize=22)),
        y=alt.Y("value:Q", title=None, axis=None, scale=alt.Scale(domain=[0, band_height])),
        color=alt.Color(
            "color_label:N",
            scale=color_scale,
            legend=alt.Legend(
                title="Intensity",
                orient="right",
                titleFontSize=18,
                labelFontSize=16,
                symbolSize=200,
                fillColor=ELEVATED_BG,
                strokeColor=INK_SOFT,
                labelColor=INK_SOFT,
                titleColor=INK,
            ),
        ),
        tooltip=["date:T", "series:N", "value:Q"],
        order=alt.Order("band:O"),
    )
    .properties(width=1400, height=80)
    .facet(
        row=alt.Row(
            "series:N",
            title=None,
            header=alt.Header(labelFontSize=16, labelAngle=0, labelAlign="left", labelPadding=10),
        )
    )
    .properties(
        title=alt.Title("horizon-basic · altair · anyplot.ai", fontSize=28, anchor="start", offset=20, color=INK)
    )
    .configure_facet(spacing=5)
    .configure_view(strokeWidth=0, fill=PAGE_BG)
    .configure_axis(
        domainColor=INK_SOFT, tickColor=INK_SOFT, gridColor=INK, gridOpacity=0.10, labelColor=INK_SOFT, titleColor=INK
    )
)

chart.save(f"plot-{THEME}.png", scale_factor=3.0)
chart.save(f"plot-{THEME}.html")

Part of Horizon Chart on anyplot.ai.

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