feat(altair): implement bar-basic (#617)

## Implementation: `bar-basic` - altair

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

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

**Parent Issue:** #612

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20150015719)*

---------

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

Author: github-actions[bot]

plots/bar-basic/implementations/altair.py

@@ -0,0 +1,39 @@
+"""
+bar-basic: Basic Bar Chart
+Library: altair
+"""
+
+import altair as alt
+import pandas as pd
+
+
+# Data
+data = pd.DataFrame(
+    {
+        "category": ["Electronics", "Clothing", "Home & Garden", "Sports", "Books", "Toys", "Food"],
+        "value": [45200, 32100, 28400, 21800, 18500, 15200, 12300],
+    }
+)
+
+# Chart
+chart = (
+    alt.Chart(data)
+    .mark_bar(color="#306998", cornerRadiusTopLeft=3, cornerRadiusTopRight=3)
+    .encode(
+        x=alt.X(
+            "category:N",
+            title="Product Category",
+            sort="-y",
+            axis=alt.Axis(labelAngle=-45, labelFontSize=16, titleFontSize=20),
+        ),
+        y=alt.Y("value:Q", title="Sales ($)", axis=alt.Axis(labelFontSize=16, titleFontSize=20)),
+        tooltip=[alt.Tooltip("category:N", title="Category"), alt.Tooltip("value:Q", title="Sales", format="$,.0f")],
+    )
+    .properties(width=1600, height=900, title=alt.Title(text="Product Sales by Category", fontSize=24))
+    .configure_view(strokeWidth=0)
+    .configure_axis(grid=True, gridOpacity=0.3)
+)
+
+# Save as PNG and HTML
+chart.save("plot.png", scale_factor=3.0)
+chart.save("plot.html")

plots/bar-basic/metadata/altair.yaml

@@ -0,0 +1,18 @@
+# Per-library metadata for altair implementation of bar-basic
+# Auto-generated by impl-generate.yml
+
+library: altair
+specification_id: bar-basic
+
+preview_url: https://storage.googleapis.com/pyplots-images/plots/bar-basic/altair/plot.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/bar-basic/altair/plot.html
+
+current:
+  version: 0
+  generated_at: 2025-12-11T22:54:47Z
+  generated_by: claude-opus-4-5-20251101
+  workflow_run: 20150015719
+  issue: 612
+  quality_score: 92
+
+history: []