Annotated Line Plot with Event Markers — Altair

A line plot with annotations at key points marking important events or milestones. This visualization enhances time series data by highlighting significant occurrences such as product launches, policy changes, or market events directly on the chart. Vertical lines, markers, and text labels draw attention to specific moments in time, making it easy to correlate data trends with real-world events.

Annotated Line Plot with Event Markers rendered with Altair

Python source (Altair)

""" anyplot.ai
line-annotated-events: Annotated Line Plot with Event Markers
Library: altair 6.1.0 | Python 3.13.13
Quality: 93/100 | Updated: 2026-05-16
"""

import os
import sys


sys.path.pop(0)

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


# Theme tokens
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"

BRAND = "#009E73"  # Okabe-Ito position 1 (bluish green)
EVENT_COLOR = "#C475FD"  # Okabe-Ito position 2 (vermillion)

# Data
np.random.seed(42)

# Generate daily stock price data for one year
dates = pd.date_range(start="2024-01-01", periods=250, freq="B")  # Business days
base_price = 150
returns = np.random.normal(0.0005, 0.015, len(dates))
prices = base_price * np.exp(np.cumsum(returns))

df = pd.DataFrame({"date": dates, "price": prices})

# Define key events (quarterly earnings and other milestones)
events = pd.DataFrame(
    {
        "event_date": pd.to_datetime(
            ["2024-02-15", "2024-04-25", "2024-06-10", "2024-07-24", "2024-09-18", "2024-11-05"]
        ),
        "event_label": [
            "Q4 Earnings",
            "Q1 Earnings",
            "Product Launch",
            "Q2 Earnings",
            "Analyst Upgrade",
            "Q3 Earnings",
        ],
        "event_type": ["Earnings", "Earnings", "Product", "Earnings", "Analyst", "Earnings"],
    }
)

# Get y positions for event labels (alternating heights to avoid overlap)
events["y_position"] = [
    prices.max() * 1.08,
    prices.max() * 1.02,
    prices.max() * 1.08,
    prices.max() * 1.02,
    prices.max() * 1.08,
    prices.max() * 1.02,
]

# Base line chart for stock price
line = (
    alt.Chart(df)
    .mark_line(strokeWidth=3, color=BRAND)
    .encode(
        x=alt.X("date:T", title="Date"),
        y=alt.Y("price:Q", title="Stock Price ($)", scale=alt.Scale(domain=[prices.min() * 0.95, prices.max() * 1.12])),
    )
)

# Get the price range for dynamic rule scaling
price_min = prices.min() * 0.95
price_max = prices.max() * 1.12

# Vertical rule marks for events (dynamic y2 based on price range)
rules = (
    alt.Chart(events)
    .mark_rule(strokeWidth=2, strokeDash=[6, 4], color=EVENT_COLOR, opacity=0.7)
    .encode(x=alt.X("event_date:T"), y=alt.value(price_min), y2=alt.value(price_max))
)

# Event markers (points at the top)
markers = (
    alt.Chart(events)
    .mark_point(size=300, filled=True, color=EVENT_COLOR, stroke=INK, strokeWidth=2)
    .encode(x=alt.X("event_date:T"), y=alt.Y("y_position:Q"), tooltip=["event_label:N", "event_type:N", "event_date:T"])
)

# Event labels
labels = (
    alt.Chart(events)
    .mark_text(align="center", baseline="bottom", fontSize=14, fontWeight="bold", color=INK, dy=-15)
    .encode(x=alt.X("event_date:T"), y=alt.Y("y_position:Q"), text="event_label:N")
)

# Combine all layers
chart = (
    alt.layer(line, rules, markers, labels)
    .properties(
        width=1600,
        height=900,
        background=PAGE_BG,
        title=alt.Title("line-annotated-events · altair · anyplot.ai", fontSize=28, anchor="middle"),
    )
    .configure_view(fill=PAGE_BG, stroke=INK_SOFT, strokeWidth=0)
    .configure_axis(
        domainColor=INK_SOFT,
        tickColor=INK_SOFT,
        gridColor=INK,
        gridOpacity=0.10,
        labelColor=INK_SOFT,
        labelFontSize=18,
        titleColor=INK,
        titleFontSize=22,
    )
    .configure_title(color=INK, fontSize=28)
    .configure_legend(
        fillColor=ELEVATED_BG,
        strokeColor=INK_SOFT,
        labelColor=INK_SOFT,
        labelFontSize=16,
        titleColor=INK,
        titleFontSize=18,
    )
)

# Save as PNG and HTML
chart.save(f"plot-{THEME}.png", scale_factor=3.0)
chart.interactive().save(f"plot-{THEME}.html")

Part of Annotated Line Plot with Event Markers on anyplot.ai.

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