feat(altair): implement facet-grid (#6518)

## Implementation: `facet-grid` - python/altair

Implements the **python/altair** version of `facet-grid`.

**File:** `plots/facet-grid/implementations/python/altair.py`

**Parent Issue:** #2696

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/25776543142)*

---------

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/facet-grid/implementations/python/altair.py

@@ -1,14 +1,25 @@
-""" pyplots.ai
+""" anyplot.ai
 facet-grid: Faceted Grid Plot
-Library: altair 6.0.0 | Python 3.13.11
-Quality: 91/100 | Created: 2025-12-30
+Library: altair 6.1.0 | Python 3.13.13
+Quality: 91/100 | Updated: 2026-05-13
 """
 
+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"
+
+OKABE_ITO = ["#009E73", "#D55E00", "#0072B2", "#CC79A7", "#E69F00", "#56B4E9", "#F0E442"]
+
 # Data - Crop yield across different soil types and seasons
 np.random.seed(42)
 
@@ -21,11 +32,8 @@
 for soil in soil_types:
     for season in seasons:
         for crop in crop_types:
-            # Base yield varies by soil type
             base_yield = {"Sandy": 3.5, "Clay": 4.0, "Loam": 5.0}[soil]
-            # Season affects yield
             season_mult = {"Spring": 0.9, "Summer": 1.1, "Fall": 1.0}[season]
-            # Crop type affects yield and water needs
             crop_base = {"Wheat": 3.8, "Corn": 4.5, "Soybean": 3.2}[crop]
 
             yield_val = np.random.normal(base_yield * season_mult + crop_base, 0.8, n_per_group)
@@ -44,31 +52,49 @@
 
 df = pd.DataFrame(data)
 
-# Create faceted chart with scatter plots - facet by soil and season, color by crop
+# Create faceted chart with scatter plots
 chart = (
     alt.Chart(df)
-    .mark_circle(size=100, opacity=0.7)
+    .mark_circle(size=150, opacity=0.7)
     .encode(
         x=alt.X("Water Usage (mm):Q", scale=alt.Scale(zero=False)),
         y=alt.Y("Yield (tons/ha):Q", scale=alt.Scale(zero=False)),
-        color=alt.Color("Crop:N", scale=alt.Scale(domain=crop_types, range=["#306998", "#FFD43B", "#4CAF50"])),
+        color=alt.Color("Crop:N", scale=alt.Scale(domain=crop_types, range=OKABE_ITO[:3])),
         tooltip=["Yield (tons/ha)", "Water Usage (mm)", "Soil Type", "Season", "Crop"],
     )
     .properties(width=320, height=260)
     .facet(
         column=alt.Column(
-            "Season:N", header=alt.Header(titleFontSize=20, labelFontSize=16), sort=["Spring", "Summer", "Fall"]
+            "Season:N", header=alt.Header(titleFontSize=22, labelFontSize=18), sort=["Spring", "Summer", "Fall"]
         ),
         row=alt.Row(
-            "Soil Type:N", header=alt.Header(titleFontSize=20, labelFontSize=16), sort=["Sandy", "Clay", "Loam"]
+            "Soil Type:N", header=alt.Header(titleFontSize=22, labelFontSize=18), sort=["Sandy", "Clay", "Loam"]
         ),
     )
-    .configure_axis(labelFontSize=14, titleFontSize=16)
-    .configure_legend(titleFontSize=18, labelFontSize=16, symbolSize=180)
-    .configure_title(fontSize=24)
-    .properties(title="facet-grid · altair · pyplots.ai")
+    .configure_axis(
+        labelFontSize=16,
+        titleFontSize=20,
+        domainColor=INK_SOFT,
+        tickColor=INK_SOFT,
+        gridColor=INK,
+        gridOpacity=0.10,
+        labelColor=INK_SOFT,
+        titleColor=INK,
+    )
+    .configure_legend(
+        titleFontSize=18,
+        labelFontSize=16,
+        symbolSize=180,
+        fillColor=ELEVATED_BG,
+        strokeColor=INK_SOFT,
+        labelColor=INK_SOFT,
+        titleColor=INK,
+    )
+    .configure_title(fontSize=28, color=INK)
+    .configure_view(fill=PAGE_BG, stroke=INK_SOFT)
+    .properties(title="facet-grid · altair · anyplot.ai", background=PAGE_BG)
 )
 
 # Save as PNG and HTML
-chart.save("plot.png", scale_factor=3.0)
-chart.save("plot.html")
+chart.save(f"plot-{THEME}.png", scale_factor=3.0)
+chart.save(f"plot-{THEME}.html")