feat(altair): implement line-basic (#5540)

## Implementation: `line-basic` - python/altair

Implements the **python/altair** version of `line-basic`.

**File:** `plots/line-basic/implementations/python/altair.py`

**Parent Issue:** #653

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:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/25117106624)*

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Markus Neusinger <2921697+MarkusNeusinger@users.noreply.github.com>

Author: github-actions[bot]

plots/line-basic/implementations/python/altair.py

@@ -1,40 +1,70 @@
-""" pyplots.ai
+""" anyplot.ai
 line-basic: Basic Line Plot
-Library: altair 6.0.0 | Python 3.13.11
-Quality: 91/100 | Created: 2025-12-23
+Library: altair 6.1.0 | Python 3.13.13
+Quality: 86/100 | Updated: 2026-04-29
 """
 
+import os
+
 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"
+
 # Data - Monthly temperature readings over a year
 np.random.seed(42)
 months = pd.date_range(start="2024-01-01", periods=12, freq="MS")
-# Simulate temperature pattern (cold in winter, warm in summer)
 base_temp = 15 + 12 * np.sin((np.arange(12) - 3) * np.pi / 6)
 temperature = base_temp + np.random.randn(12) * 2
-
 df = pd.DataFrame({"Month": months, "Temperature": temperature})
 
 # Plot
-chart = (
+tooltip_enc = [
+    alt.Tooltip("Month:T", title="Month", format="%B %Y"),
+    alt.Tooltip("Temperature:Q", title="Temperature (°C)", format=".1f"),
+]
+
+line = (
     alt.Chart(df)
-    .mark_line(strokeWidth=4, color="#306998")
-    .encode(
-        x=alt.X("Month:T", title="Month", axis=alt.Axis(labelFontSize=18, titleFontSize=22)),
-        y=alt.Y("Temperature:Q", title="Temperature (°C)", axis=alt.Axis(labelFontSize=18, titleFontSize=22)),
-    )
-    .properties(width=1600, height=900, title=alt.Title("line-basic · altair · pyplots.ai", fontSize=28))
+    .mark_line(strokeWidth=4, color=BRAND)
+    .encode(x=alt.X("Month:T", title="Month"), y=alt.Y("Temperature:Q", title="Temperature (°C)"))
 )
 
-# Add points to enhance visibility
-points = alt.Chart(df).mark_point(size=200, color="#306998", filled=True).encode(x="Month:T", y="Temperature:Q")
+points = (
+    alt.Chart(df)
+    .mark_point(size=200, color=BRAND, filled=True)
+    .encode(x="Month:T", y="Temperature:Q", tooltip=tooltip_enc)
+)
 
-# Combine line and points with subtle grid
-final_chart = (chart + points).configure_axis(gridColor="#E0E0E0", gridOpacity=0.3).configure_view(strokeWidth=0)
+chart = (
+    (line + points)
+    .interactive()
+    .properties(
+        width=1600, height=900, title=alt.Title("line-basic · altair · anyplot.ai", fontSize=28), background=PAGE_BG
+    )
+    .configure_view(fill=PAGE_BG, stroke=INK_SOFT)
+    .configure_axis(
+        domainColor=INK_SOFT,
+        tickColor=INK_SOFT,
+        gridColor=INK,
+        gridOpacity=0.15,
+        labelColor=INK_SOFT,
+        titleColor=INK,
+        labelFontSize=18,
+        titleFontSize=22,
+    )
+    .configure_title(color=INK, fontSize=28)
+    .configure_legend(fillColor=ELEVATED_BG, strokeColor=INK_SOFT, labelColor=INK_SOFT, titleColor=INK)
+)
 
 # Save
-final_chart.save("plot.png", scale_factor=3.0)
-final_chart.save("plot.html")
+chart.save(f"plot-{THEME}.png", scale_factor=3.0)
+chart.save(f"plot-{THEME}.html")