feat(altair): implement confusion-matrix (#2286)

## Implementation: `confusion-matrix` - altair

Implements the **altair** version of `confusion-matrix`.

**File:** `plots/confusion-matrix/implementations/altair.py`

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20526592033)*

---------

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/confusion-matrix/implementations/altair.py

@@ -0,0 +1,78 @@
+""" pyplots.ai
+confusion-matrix: Confusion Matrix Heatmap
+Library: altair 6.0.0 | Python 3.13.11
+Quality: 92/100 | Created: 2025-12-26
+"""
+
+import altair as alt
+import numpy as np
+import pandas as pd
+
+
+# Data - Multi-class classification results
+np.random.seed(42)
+class_names = ["Dog", "Cat", "Bird", "Fish"]
+n_classes = len(class_names)
+
+# Create a realistic confusion matrix with clear patterns
+# Most predictions on diagonal (correct), some off-diagonal (errors)
+confusion = np.array(
+    [
+        [85, 10, 3, 2],  # Dogs: mostly correct, some confused with cats
+        [12, 78, 6, 4],  # Cats: mostly correct, some confused with dogs
+        [5, 8, 82, 5],  # Birds: mostly correct, some confusion
+        [2, 4, 3, 91],  # Fish: very distinct, high accuracy
+    ]
+)
+
+# Create long-form DataFrame for Altair
+rows = []
+for i, true_class in enumerate(class_names):
+    for j, pred_class in enumerate(class_names):
+        rows.append(
+            {
+                "True Label": true_class,
+                "Predicted Label": pred_class,
+                "Count": confusion[i, j],
+            }
+        )
+
+df = pd.DataFrame(rows)
+
+# Base heatmap with rectangles
+base = alt.Chart(df).encode(
+    x=alt.X("Predicted Label:N", sort=class_names, axis=alt.Axis(labelAngle=0, labelFontSize=20, titleFontSize=24)),
+    y=alt.Y("True Label:N", sort=class_names, axis=alt.Axis(labelFontSize=20, titleFontSize=24)),
+)
+
+# Heatmap cells
+heatmap = base.mark_rect(stroke="white", strokeWidth=2).encode(
+    color=alt.Color(
+        "Count:Q",
+        scale=alt.Scale(scheme="blues"),
+        legend=alt.Legend(title="Count", titleFontSize=18, labelFontSize=16, gradientLength=300, gradientThickness=25),
+    )
+)
+
+# Text annotations - white on dark cells, dark on light cells
+text = base.mark_text(fontSize=28, fontWeight="bold").encode(
+    text="Count:Q", color=alt.condition(alt.datum.Count > 50, alt.value("white"), alt.value("#306998"))
+)
+
+# Combine heatmap and text
+chart = (
+    (heatmap + text)
+    .properties(
+        width=1000,
+        height=1000,
+        title=alt.Title("confusion-matrix · altair · pyplots.ai", fontSize=32, anchor="middle", offset=20),
+    )
+    .configure_view(strokeWidth=0)
+    .configure_axis(domainWidth=0)
+)
+
+# Save as PNG (1000 * 3.6 = 3600 for square format)
+chart.save("plot.png", scale_factor=3.6)
+
+# Save interactive HTML version
+chart.save("plot.html")

plots/confusion-matrix/metadata/altair.yaml

@@ -0,0 +1,25 @@
+library: altair
+specification_id: confusion-matrix
+created: '2025-12-26T17:37:13Z'
+updated: '2025-12-26T17:41:34Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20526592033
+issue: 0
+python_version: 3.13.11
+library_version: 6.0.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/confusion-matrix/altair/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/confusion-matrix/altair/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/confusion-matrix/altair/plot.html
+quality_score: 92
+review:
+  strengths:
+  - Excellent text contrast logic using alt.condition to switch between white and
+    dark text based on cell value
+  - Clean declarative Altair style with proper encoding types (:N, :Q)
+  - Well-structured long-form DataFrame transformation for Altair grammar
+  - Good use of layered composition (heatmap + text)
+  - Square aspect ratio appropriate for confusion matrix
+  - White cell borders enhance visual clarity
+  weaknesses:
+  - Missing interactive tooltips which are a distinctive Altair feature
+  - Legend positioning could be optimized to be closer to the plot