feat(altair): implement donut-basic (#5340)

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

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

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

**Parent Issue:** #733

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/24873943198)*

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

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

@@ -1,70 +1,85 @@
-""" pyplots.ai
+""" anyplot.ai
 donut-basic: Basic Donut Chart
-Library: altair 6.0.0 | Python 3.13.11
-Quality: 92/100 | Created: 2025-12-23
+Library: altair 6.1.0 | Python 3.14.4
+Quality: 91/100 | Updated: 2026-04-24
 """
 
-import altair as alt
-import pandas as pd
+import importlib
+import os
+import sys
 
 
-# Data - Budget allocation by department
+# Drop script directory from sys.path so the `altair` package resolves, not this file
+sys.path[:] = [p for p in sys.path if os.path.abspath(p or ".") != os.path.dirname(os.path.abspath(__file__))]
+alt = importlib.import_module("altair")
+pd = importlib.import_module("pandas")
+
+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"]
+
 data = pd.DataFrame(
-    {"category": ["Marketing", "Development", "Operations", "Sales", "Support"], "value": [28, 35, 18, 12, 7]}
+    {"category": ["Engineering", "Marketing", "Operations", "Sales", "Support"], "value": [480, 210, 155, 125, 55]}
 )
 
-# Calculate percentages for labels
-total = data["value"].sum()
+total = int(data["value"].sum())
 data["percentage"] = (data["value"] / total * 100).round(1)
 data["label"] = data["percentage"].astype(str) + "%"
 
-# Create donut chart using arc mark
-chart = (
+arc = (
     alt.Chart(data)
-    .mark_arc(innerRadius=120, outerRadius=280, stroke="white", strokeWidth=3)
+    .mark_arc(innerRadius=260, outerRadius=520, stroke=PAGE_BG, strokeWidth=4)
     .encode(
         theta=alt.Theta(field="value", type="quantitative", stack=True),
         color=alt.Color(
             field="category",
             type="nominal",
-            scale=alt.Scale(
-                domain=["Marketing", "Development", "Operations", "Sales", "Support"],
-                range=["#306998", "#FFD43B", "#4B8BBE", "#646464", "#8FBC8F"],
+            scale=alt.Scale(domain=list(data["category"]), range=OKABE_ITO),
+            legend=alt.Legend(
+                title="Department", titleFontSize=22, labelFontSize=18, orient="right", symbolSize=400, padding=16
             ),
-            legend=alt.Legend(title="Category", titleFontSize=20, labelFontSize=16, orient="right", symbolSize=300),
         ),
+        order=alt.Order(field="value", sort="descending"),
         tooltip=[
-            alt.Tooltip("category:N", title="Category"),
-            alt.Tooltip("value:Q", title="Budget ($M)"),
-            alt.Tooltip("percentage:Q", title="Percentage", format=".1f"),
+            alt.Tooltip("category:N", title="Department"),
+            alt.Tooltip("value:Q", title="Budget ($K)"),
+            alt.Tooltip("percentage:Q", title="Share (%)", format=".1f"),
         ],
     )
-    .properties(
-        width=1600, height=900, title=alt.Title(text="donut-basic · altair · pyplots.ai", fontSize=28, anchor="middle")
-    )
 )
 
-# Add percentage labels on segments
-text = (
+labels = (
     alt.Chart(data)
-    .mark_text(radius=200, fontSize=20, fontWeight="bold")
+    .mark_text(radius=390, fontSize=22, fontWeight="bold", color="#FFFFFF")
     .encode(
         theta=alt.Theta(field="value", type="quantitative", stack=True),
+        order=alt.Order(field="value", sort="descending"),
         text=alt.Text("label:N"),
-        color=alt.value("white"),
     )
 )
 
-# Add center text showing total
-center_text = (
-    alt.Chart(pd.DataFrame({"text": [f"Total: ${total}M"]}))
-    .mark_text(fontSize=36, fontWeight="bold", color="#306998")
-    .encode(text="text:N")
+center = (
+    alt.Chart(pd.DataFrame({"line": ["Total budget", f"${total:,}K"], "y": [0.08, -0.08]}))
+    .mark_text(fontSize=36, fontWeight="bold", color=INK, align="center")
+    .encode(y=alt.Y("y:Q", axis=None, scale=alt.Scale(domain=[-1, 1])), text="line:N")
 )
 
-# Combine layers
-final_chart = alt.layer(chart, text, center_text).configure_view(strokeWidth=0)
+final_chart = (
+    alt.layer(arc, labels, center)
+    .properties(
+        width=1200,
+        height=1200,
+        background=PAGE_BG,
+        title=alt.Title(text="donut-basic · altair · anyplot.ai", fontSize=28, anchor="middle", color=INK, offset=20),
+        padding={"left": 40, "right": 40, "top": 20, "bottom": 20},
+    )
+    .configure_view(fill=PAGE_BG, stroke=None)
+    .configure_legend(fillColor=ELEVATED_BG, strokeColor=INK_SOFT, labelColor=INK_SOFT, titleColor=INK, cornerRadius=6)
+)
 
-# Save outputs
-final_chart.save("plot.png", scale_factor=3.0)
-final_chart.save("plot.html")
+final_chart.save(f"plot-{THEME}.png", scale_factor=3.0)
+final_chart.save(f"plot-{THEME}.html")