diff --git a/plots/scatter-constellation-diagram/implementations/altair.py b/plots/scatter-constellation-diagram/implementations/altair.py
new file mode 100644
index 0000000000..6bf7201cee
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+++ b/plots/scatter-constellation-diagram/implementations/altair.py
@@ -0,0 +1,135 @@
+""" pyplots.ai
+scatter-constellation-diagram: Digital Modulation Constellation Diagram
+Library: altair 6.0.0 | Python 3.14.3
+Quality: 87/100 | Created: 2026-03-17
+"""
+
+import altair as alt
+import numpy as np
+import pandas as pd
+
+
+# Data
+np.random.seed(42)
+
+ideal_vals = [-3, -1, 1, 3]
+ideal_i, ideal_q = np.meshgrid(ideal_vals, ideal_vals)
+ideal_i = ideal_i.flatten()
+ideal_q = ideal_q.flatten()
+
+n_symbols = 1000
+symbol_indices = np.random.randint(0, 16, size=n_symbols)
+
+snr_db = 20
+snr_linear = 10 ** (snr_db / 10)
+signal_power = np.mean(ideal_i**2 + ideal_q**2)
+noise_std = np.sqrt(signal_power / snr_linear)
+
+received_i = ideal_i[symbol_indices] + np.random.normal(0, noise_std, n_symbols)
+received_q = ideal_q[symbol_indices] + np.random.normal(0, noise_std, n_symbols)
+
+error_vectors = np.sqrt((received_i - ideal_i[symbol_indices]) ** 2 + (received_q - ideal_q[symbol_indices]) ** 2)
+rms_signal = np.sqrt(signal_power)
+evm_pct = np.sqrt(np.mean(error_vectors**2)) / rms_signal * 100
+
+# Per-symbol error magnitude for color encoding
+df_received = pd.DataFrame(
+    {
+        "I": received_i,
+        "Q": received_q,
+        "Error Magnitude": error_vectors,
+        "Nearest I": ideal_i[symbol_indices],
+        "Nearest Q": ideal_q[symbol_indices],
+    }
+)
+df_ideal = pd.DataFrame({"I": ideal_i, "Q": ideal_q, "label": "Ideal"})
+
+# Decision boundaries
+boundary_vals = [-4, -2, 0, 2, 4]
+boundary_h = pd.DataFrame([{"x": -5.2, "x2": 5.2, "y": v} for v in boundary_vals])
+boundary_v = pd.DataFrame([{"y": -5.2, "y2": 5.2, "x": v} for v in boundary_vals])
+
+# EVM annotation
+df_evm = pd.DataFrame({"I": [4.2], "Q": [4.8], "label": [f"EVM = {evm_pct:.1f}%"]})
+
+# Selection for interactive nearest-point highlighting
+nearest = alt.selection_point(on="pointerover", nearest=True, fields=["I", "Q"], empty=False)
+
+# Plot layers
+scale_x = alt.Scale(domain=[-5.5, 5.5], nice=False)
+scale_y = alt.Scale(domain=[-5.5, 5.5], nice=False)
+
+received_layer = (
+    alt.Chart(df_received)
+    .mark_circle(size=45)
+    .encode(
+        x=alt.X("I:Q", title="In-Phase (I)", scale=scale_x),
+        y=alt.Y("Q:Q", title="Quadrature (Q)", scale=scale_y),
+        color=alt.Color(
+            "Error Magnitude:Q",
+            scale=alt.Scale(scheme="viridis"),
+            legend=alt.Legend(
+                title="Error Mag.", titleFontSize=16, labelFontSize=14, orient="right", gradientLength=200
+            ),
+        ),
+        opacity=alt.condition(nearest, alt.value(0.85), alt.value(0.3)),
+        size=alt.condition(nearest, alt.value(120), alt.value(45)),
+        tooltip=[
+            alt.Tooltip("I:Q", format=".3f"),
+            alt.Tooltip("Q:Q", format=".3f"),
+            alt.Tooltip("Error Magnitude:Q", format=".3f", title="Error"),
+            alt.Tooltip("Nearest I:Q", format=".0f", title="Ideal I"),
+            alt.Tooltip("Nearest Q:Q", format=".0f", title="Ideal Q"),
+        ],
+    )
+    .add_params(nearest)
+)
+
+ideal_layer = (
+    alt.Chart(df_ideal)
+    .mark_point(size=400, filled=False, strokeWidth=3.5)
+    .encode(
+        x="I:Q",
+        y="Q:Q",
+        color=alt.value("#E45756"),
+        shape=alt.value("cross"),
+        tooltip=[alt.Tooltip("I:Q", format=".0f", title="Ideal I"), alt.Tooltip("Q:Q", format=".0f", title="Ideal Q")],
+    )
+)
+
+h_rules = (
+    alt.Chart(boundary_h)
+    .mark_rule(strokeDash=[8, 5], strokeWidth=1, opacity=0.35)
+    .encode(x=alt.X("x:Q", scale=scale_x), x2="x2:Q", y=alt.Y("y:Q", scale=scale_y), color=alt.value("#AAAAAA"))
+)
+
+v_rules = (
+    alt.Chart(boundary_v)
+    .mark_rule(strokeDash=[8, 5], strokeWidth=1, opacity=0.35)
+    .encode(y=alt.Y("y:Q", scale=scale_y), y2="y2:Q", x=alt.X("x:Q", scale=scale_x), color=alt.value("#AAAAAA"))
+)
+
+evm_label = (
+    alt.Chart(df_evm)
+    .mark_text(fontSize=22, fontWeight="bold", align="right", font="monospace")
+    .encode(x="I:Q", y="Q:Q", text="label:N", color=alt.value("#222222"))
+)
+
+chart = (
+    alt.layer(h_rules, v_rules, received_layer, ideal_layer, evm_label)
+    .properties(
+        width=1020,
+        height=1100,
+        title=alt.Title(
+            "scatter-constellation-diagram \u00b7 altair \u00b7 pyplots.ai", fontSize=28, anchor="middle", offset=12
+        ),
+    )
+    .configure_axis(
+        labelFontSize=18, titleFontSize=22, tickSize=8, domainColor="#666666", tickColor="#888888", grid=False
+    )
+    .configure_view(strokeWidth=0)
+)
+
+# Save
+chart.save("plot.png", scale_factor=3.0)
+chart.save("plot.html")
diff --git a/plots/scatter-constellation-diagram/metadata/altair.yaml b/plots/scatter-constellation-diagram/metadata/altair.yaml
new file mode 100644
index 0000000000..b2d6519f28
--- /dev/null
+++ b/plots/scatter-constellation-diagram/metadata/altair.yaml
@@ -0,0 +1,235 @@
+library: altair
+specification_id: scatter-constellation-diagram
+created: '2026-03-17T23:22:50Z'
+updated: '2026-03-17T23:56:47Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 23220948598
+issue: 4562
+python_version: 3.14.3
+library_version: 6.0.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/scatter-constellation-diagram/altair/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/scatter-constellation-diagram/altair/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/scatter-constellation-diagram/altair/plot.html
+quality_score: 87
+review:
+  strengths:
+  - Perfect spec compliance with all required features correctly implemented (16-QAM,
+    ideal crosses, received dots, decision boundaries, EVM, axis labels, title format)
+  - Viridis color encoding of error magnitude adds a meaningful analytical dimension
+    beyond spec requirements
+  - Interactive features (selection highlighting, conditional encoding, tooltips)
+    showcase Altair distinctive capabilities
+  - Mathematically correct EVM calculation consistent with 20 dB SNR
+  - Clean, well-structured code following KISS principles
+  weaknesses: []
+  image_description: The plot displays a 16-QAM constellation diagram on a near-square
+    canvas (1020x1100). Sixteen ideal constellation points appear as red/salmon cross
+    markers (#E45756) arranged in a 4x4 grid at I/Q coordinates +/-1 and +/-3. Approximately
+    1000 received symbols are shown as semi-transparent circles color-encoded by error
+    magnitude using the viridis colormap — dark purple clusters near ideal points
+    (low error) transitioning to teal/green and yellow for higher-error outliers.
+    Dashed gray decision boundary lines are drawn at I/Q values -4, -2, 0, 2, 4 on
+    both axes. The title "scatter-constellation-diagram · altair · pyplots.ai" is
+    displayed at top center in 28pt font. Axes are labeled "In-Phase (I)" (x) and
+    "Quadrature (Q)" (y) with tick labels at 18pt. A bold monospace "EVM = 14.0%"
+    annotation sits in the upper-right quadrant. A viridis color legend labeled "Error
+    Mag." appears on the right side. The background is white with no default grid;
+    view border stroke is removed. The overall layout is clean and approximately square
+    with the constellation geometry well-preserved.
+  criteria_checklist:
+    visual_quality:
+      score: 27
+      max: 30
+      items:
+      - id: VQ-01
+        name: Text Legibility
+        score: 7
+        max: 8
+        passed: true
+        comment: 'Font sizes explicitly set: title=28, axis titles=22, ticks=18, EVM=22,
+          legend title=16, legend labels=14. All readable. Legend labels slightly
+          small at 14pt.'
+      - id: VQ-02
+        name: No Overlap
+        score: 6
+        max: 6
+        passed: true
+        comment: No text or element overlap anywhere in the plot.
+      - id: VQ-03
+        name: Element Visibility
+        score: 5
+        max: 6
+        passed: true
+        comment: Size=45, opacity=0.3 for 1000 points is within guidelines. Ideal
+          crosses at size=400 are prominent. Some peripheral points faint at base
+          opacity.
+      - id: VQ-04
+        name: Color Accessibility
+        score: 4
+        max: 4
+        passed: true
+        comment: Viridis colormap is perceptually uniform and colorblind-safe. Red
+          crosses provide strong luminance contrast.
+      - id: VQ-05
+        name: Layout & Canvas
+        score: 3
+        max: 4
+        passed: true
+        comment: Canvas 1020x1100 at scale 3.0 = 3060x3300 px. Improved from 2700x2700
+          but below 3600x3600 target. Not perfectly square.
+      - id: VQ-06
+        name: Axis Labels & Title
+        score: 2
+        max: 2
+        passed: true
+        comment: In-Phase (I) and Quadrature (Q) are descriptive with standard domain
+          notation.
+    design_excellence:
+      score: 13
+      max: 20
+      items:
+      - id: DE-01
+        name: Aesthetic Sophistication
+        score: 5
+        max: 8
+        passed: true
+        comment: Viridis error magnitude encoding is thoughtful. Red crosses contrast
+          well. Monospace EVM annotation. Above defaults but not publication-level.
+      - id: DE-02
+        name: Visual Refinement
+        score: 4
+        max: 6
+        passed: true
+        comment: Grid disabled, view stroke removed, custom domain/tick colors, dashed
+          decision boundaries as structural context.
+      - id: DE-03
+        name: Data Storytelling
+        score: 4
+        max: 6
+        passed: true
+        comment: Error magnitude color encoding creates visual hierarchy. EVM annotation
+          quantifies modulation quality. Viewer immediately understands signal integrity.
+    spec_compliance:
+      score: 15
+      max: 15
+      items:
+      - id: SC-01
+        name: Plot Type
+        score: 5
+        max: 5
+        passed: true
+        comment: Correct I/Q scatter constellation diagram for 16-QAM.
+      - id: SC-02
+        name: Required Features
+        score: 4
+        max: 4
+        passed: true
+        comment: 'All spec features present: ideal crosses, received dots, decision
+          boundaries, equal aspect, EVM annotation, axis labels.'
+      - id: SC-03
+        name: Data Mapping
+        score: 3
+        max: 3
+        passed: true
+        comment: I mapped to x-axis, Q to y-axis, symmetric limits centered at origin.
+      - id: SC-04
+        name: Title & Legend
+        score: 3
+        max: 3
+        passed: true
+        comment: Title follows exact format. Legend shows Error Mag. which is appropriate.
+    data_quality:
+      score: 14
+      max: 15
+      items:
+      - id: DQ-01
+        name: Feature Coverage
+        score: 5
+        max: 6
+        passed: true
+        comment: Shows all 16 ideal points, noisy received symbols, error magnitude
+          via color, decision boundaries, EVM. Color adds analytical depth.
+      - id: DQ-02
+        name: Realistic Context
+        score: 5
+        max: 5
+        passed: true
+        comment: 16-QAM is a real digital modulation scheme used in Wi-Fi, 5G NR,
+          DVB. Neutral technical domain.
+      - id: DQ-03
+        name: Appropriate Scale
+        score: 4
+        max: 4
+        passed: true
+        comment: Standard grid values for 16-QAM, 20 dB SNR realistic, 1000 symbols
+          appropriate, EVM=14.0% consistent.
+    code_quality:
+      score: 10
+      max: 10
+      items:
+      - id: CQ-01
+        name: KISS Structure
+        score: 3
+        max: 3
+        passed: true
+        comment: 'Clean linear flow: imports, data generation, plot layers, composition,
+          save.'
+      - id: CQ-02
+        name: Reproducibility
+        score: 2
+        max: 2
+        passed: true
+        comment: np.random.seed(42) set at start.
+      - id: CQ-03
+        name: Clean Imports
+        score: 2
+        max: 2
+        passed: true
+        comment: All three imports (altair, numpy, pandas) are used.
+      - id: CQ-04
+        name: Code Elegance
+        score: 2
+        max: 2
+        passed: true
+        comment: Clean, well-structured. Proper EVM calculation with correct signal
+          processing math.
+      - id: CQ-05
+        name: Output & API
+        score: 1
+        max: 1
+        passed: true
+        comment: Saves as plot.png and plot.html. Current Altair 6.0 API.
+    library_mastery:
+      score: 8
+      max: 10
+      items:
+      - id: LM-01
+        name: Idiomatic Usage
+        score: 4
+        max: 5
+        passed: true
+        comment: 'Good declarative grammar: alt.layer(), proper encoding types, Scale/Legend/Title
+          configs, configure_axis/configure_view.'
+      - id: LM-02
+        name: Distinctive Features
+        score: 4
+        max: 5
+        passed: true
+        comment: Uses alt.selection_point for nearest-point highlighting, alt.condition
+          for dynamic opacity/size, rich formatted tooltips, HTML export. Distinctive
+          Altair features.
+  verdict: APPROVED
+impl_tags:
+  dependencies: []
+  techniques:
+  - layer-composition
+  - annotations
+  - hover-tooltips
+  - html-export
+  - custom-legend
+  patterns:
+  - data-generation
+  dataprep: []
+  styling:
+  - custom-colormap
+  - alpha-blending