feat(altair): implement scatter-basic (#5322)

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

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

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

**Parent Issue:** #611

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/24860303745)*

---------

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/scatter-basic/implementations/python/altair.py

@@ -1,7 +1,7 @@
 """ anyplot.ai
 scatter-basic: Basic Scatter Plot
 Library: altair 6.1.0 | Python 3.14.4
-Quality: 88/100 | Created: 2026-04-23
+Quality: 89/100 | Updated: 2026-04-23
 """
 
 import importlib
@@ -21,23 +21,41 @@
 PAGE_BG = "#FAF8F1" if THEME == "light" else "#1A1A17"
 INK = "#1A1A17" if THEME == "light" else "#F0EFE8"
 INK_SOFT = "#4A4A44" if THEME == "light" else "#B8B7B0"
-BRAND = "#009E73"
+INK_MUTED = "#6B6A63" if THEME == "light" else "#A8A79F"
+BRAND = "#009E73"  # Okabe-Ito position 1
 
-# Data
+# Data — study hours vs. exam scores (r ~ 0.75)
 np.random.seed(42)
 n = 180
 study_hours = np.random.uniform(1, 12, n)
-exam_scores = np.clip(40 + study_hours * 4.8 + np.random.randn(n) * 7.5, 30, 100)
+exam_scores = np.clip(38 + study_hours * 4.8 + np.random.normal(0, 7.0, n), 30, 100)
 df = pd.DataFrame({"hours": study_hours, "score": exam_scores})
 
+pearson_r = float(np.corrcoef(df["hours"], df["score"])[0, 1])
+
 # Plot
 points = (
     alt.Chart(df)
-    .mark_circle(size=180, opacity=0.7, color=BRAND, stroke=PAGE_BG, strokeWidth=0.8)
+    .mark_circle(size=170, opacity=0.7, color=BRAND, stroke=PAGE_BG, strokeWidth=0.8)
     .encode(
-        x=alt.X("hours:Q", title="Study Hours per Week", scale=alt.Scale(domain=[0, 13], nice=False)),
-        y=alt.Y("score:Q", title="Exam Score (%)", scale=alt.Scale(domain=[25, 105], nice=False)),
+        x=alt.X(
+            "hours:Q",
+            title="Study Hours per Week",
+            scale=alt.Scale(domain=[0, 13], nice=False),
+            axis=alt.Axis(tickCount=6, ticks=False, labelPadding=10, titlePadding=18),
+        ),
+        y=alt.Y(
+            "score:Q",
+            title="Exam Score (%)",
+            scale=alt.Scale(domain=[25, 105], nice=False),
+            axis=alt.Axis(tickCount=8, ticks=False, labelPadding=10, titlePadding=18),
+        ),
+        tooltip=[
+            alt.Tooltip("hours:Q", title="Study hrs / wk", format=".1f"),
+            alt.Tooltip("score:Q", title="Exam %", format=".1f"),
+        ],
     )
+    .interactive()
 )
 
 chart = (
@@ -47,19 +65,24 @@
         background=PAGE_BG,
         title=alt.Title(
             "scatter-basic · altair · anyplot.ai",
+            subtitle=f"n = {n}  ·  Pearson r = {pearson_r:.2f}",
             fontSize=28,
             fontWeight="normal",
             color=INK,
+            subtitleFontSize=16,
+            subtitleColor=INK_MUTED,
+            subtitlePadding=6,
             anchor="start",
-            offset=16,
+            offset=20,
         ),
     )
     .configure_view(fill=PAGE_BG, stroke=None)
     .configure_axis(
         labelFontSize=18,
         titleFontSize=22,
-        titlePadding=14,
+        titleFontWeight="normal",
         domainColor=INK_SOFT,
+        domainWidth=0.9,
         tickColor=INK_SOFT,
         gridColor=INK,
         gridOpacity=0.10,