feat(altair): implement scatter-basic (#615)

## Implementation: `scatter-basic` - altair

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

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

**Parent Issue:** #611

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:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20150014835)*

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Author: github-actions[bot]

plots/scatter-basic/implementations/altair.py

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+"""
+scatter-basic: Basic Scatter Plot
+Library: altair
+"""
+
+import altair as alt
+import numpy as np
+import pandas as pd
+
+
+# Data
+np.random.seed(42)
+x = np.random.randn(100) * 2 + 10
+y = x * 0.8 + np.random.randn(100) * 2
+
+df = pd.DataFrame({"x": x, "y": y})
+
+# Create chart
+chart = (
+    alt.Chart(df)
+    .mark_point(filled=True, size=100, opacity=0.7, color="#306998")
+    .encode(x=alt.X("x:Q", title="X Value"), y=alt.Y("y:Q", title="Y Value"), tooltip=["x:Q", "y:Q"])
+    .properties(width=1600, height=900, title="Basic Scatter Plot")
+    .configure_axis(labelFontSize=16, titleFontSize=20, grid=True, gridOpacity=0.3)
+    .configure_title(fontSize=20)
+)
+
+# Save as PNG and HTML
+chart.save("plot.png", scale_factor=3.0)
+chart.save("plot.html")

plots/scatter-basic/metadata/altair.yaml

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+# Per-library metadata for altair implementation of scatter-basic
+# Auto-generated by impl-generate.yml
+
+library: altair
+specification_id: scatter-basic
+
+preview_url: https://storage.googleapis.com/pyplots-images/plots/scatter-basic/altair/plot.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/scatter-basic/altair/plot.html
+
+current:
+  version: 0
+  generated_at: 2025-12-11T22:54:30Z
+  generated_by: claude-opus-4-5-20251101
+  workflow_run: 20150014835
+  issue: 611
+  quality_score: 88
+
+history: []