diff --git a/plots/subplot-mosaic/implementations/altair.py b/plots/subplot-mosaic/implementations/altair.py
new file mode 100644
index 0000000000..3a5d865536
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+++ b/plots/subplot-mosaic/implementations/altair.py
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+""" pyplots.ai
+subplot-mosaic: Mosaic Subplot Layout with Varying Sizes
+Library: altair 6.0.0 | Python 3.13.11
+Quality: 91/100 | Created: 2025-12-31
+"""
+
+import altair as alt
+import numpy as np
+import pandas as pd
+
+
+# Data - Create diverse datasets for dashboard-style mosaic layout
+np.random.seed(42)
+
+# Panel A: Wide time series (top, spanning 2 columns)
+dates = pd.date_range("2024-01-01", periods=100, freq="D")
+df_timeseries = pd.DataFrame(
+    {"date": dates, "value": np.cumsum(np.random.randn(100)) + 50, "category": "Revenue Trend"}
+)
+
+# Panel B: Small metric (top right corner)
+df_gauge = pd.DataFrame({"metric": ["Current"], "value": [78], "max_value": [100]})
+
+# Panel C: Bar chart (middle left)
+df_bars = pd.DataFrame({"region": ["North", "South", "East", "West", "Central"], "sales": [45, 38, 52, 29, 41]})
+
+# Panel D: Scatter plot (middle right, spanning 2 rows)
+n_points = 80
+df_scatter = pd.DataFrame(
+    {
+        "efficiency": np.random.uniform(60, 95, n_points),
+        "output": np.random.uniform(100, 500, n_points) + np.random.uniform(60, 95, n_points) * 3,
+        "size": np.random.uniform(20, 100, n_points),
+    }
+)
+
+# Panel E: Small bar chart (bottom left)
+df_categories = pd.DataFrame({"type": ["Type A", "Type B", "Type C"], "count": [24, 18, 31]})
+
+# Panel F: Small area chart (bottom middle)
+df_area = pd.DataFrame(
+    {
+        "hour": list(range(24)),
+        "traffic": [10, 8, 5, 4, 6, 15, 35, 55, 48, 42, 38, 45, 50, 48, 52, 60, 65, 55, 40, 30, 25, 20, 15, 12],
+    }
+)
+
+# Note: Data is small enough to not need max_rows configuration
+
+# Create individual charts with proper styling
+
+# Chart A: Wide time series (spans 2 columns at top)
+chart_a = (
+    alt.Chart(df_timeseries)
+    .mark_line(strokeWidth=3, color="#306998")
+    .encode(
+        x=alt.X("date:T", title="Date", axis=alt.Axis(labelFontSize=14, titleFontSize=16)),
+        y=alt.Y("value:Q", title="Revenue ($K)", axis=alt.Axis(labelFontSize=14, titleFontSize=16)),
+    )
+    .properties(width=700, height=200, title=alt.Title("Monthly Revenue Overview", fontSize=20))
+)
+
+# Chart B: Gauge-style metric (small, top right)
+chart_b_bg = (
+    alt.Chart(df_gauge).mark_arc(innerRadius=50, outerRadius=80, theta=3.14159, theta2=0, color="#E0E0E0").encode()
+)
+
+chart_b_value = (
+    alt.Chart(df_gauge)
+    .mark_arc(
+        innerRadius=50,
+        outerRadius=80,
+        theta=3.14159,
+        theta2=alt.expr("3.14159 - (datum.value / datum.max_value) * 3.14159"),
+        color="#306998",
+    )
+    .encode()
+)
+
+chart_b_text = (
+    alt.Chart(df_gauge)
+    .mark_text(fontSize=28, fontWeight="bold", color="#306998")
+    .encode(text=alt.Text("value:Q", format=".0f"))
+)
+
+chart_b = alt.layer(chart_b_bg, chart_b_value, chart_b_text).properties(
+    width=200, height=200, title=alt.Title("Performance Score", fontSize=18)
+)
+
+# Chart C: Bar chart (middle left)
+chart_c = (
+    alt.Chart(df_bars)
+    .mark_bar(color="#306998", cornerRadiusTopLeft=4, cornerRadiusTopRight=4)
+    .encode(
+        x=alt.X("region:N", title="Region", axis=alt.Axis(labelFontSize=14, titleFontSize=16, labelAngle=0)),
+        y=alt.Y("sales:Q", title="Sales ($K)", axis=alt.Axis(labelFontSize=14, titleFontSize=16)),
+    )
+    .properties(width=280, height=180, title=alt.Title("Sales by Region", fontSize=18))
+)
+
+# Chart D: Scatter plot (spans 2 rows on right side)
+chart_d = (
+    alt.Chart(df_scatter)
+    .mark_circle(opacity=0.7)
+    .encode(
+        x=alt.X(
+            "efficiency:Q",
+            title="Efficiency (%)",
+            scale=alt.Scale(domain=[55, 100]),
+            axis=alt.Axis(labelFontSize=14, titleFontSize=16),
+        ),
+        y=alt.Y("output:Q", title="Output (units)", axis=alt.Axis(labelFontSize=14, titleFontSize=16)),
+        size=alt.Size("size:Q", scale=alt.Scale(range=[50, 300]), legend=None),
+        color=alt.Color("efficiency:Q", scale=alt.Scale(scheme="blues"), legend=None),
+    )
+    .properties(width=320, height=400, title=alt.Title("Efficiency vs Output", fontSize=18))
+)
+
+# Chart E: Small bar chart (bottom left)
+chart_e = (
+    alt.Chart(df_categories)
+    .mark_bar(color="#FFD43B")
+    .encode(
+        x=alt.X("type:N", title=None, axis=alt.Axis(labelFontSize=12, labelAngle=0)),
+        y=alt.Y("count:Q", title="Count", axis=alt.Axis(labelFontSize=12, titleFontSize=14)),
+    )
+    .properties(width=180, height=160, title=alt.Title("By Category", fontSize=16))
+)
+
+# Chart F: Area chart (bottom middle)
+chart_f = (
+    alt.Chart(df_area)
+    .mark_area(color="#306998", opacity=0.7, line={"color": "#306998", "strokeWidth": 2})
+    .encode(
+        x=alt.X("hour:Q", title="Hour", axis=alt.Axis(labelFontSize=12, titleFontSize=14)),
+        y=alt.Y("traffic:Q", title="Traffic", axis=alt.Axis(labelFontSize=12, titleFontSize=14)),
+    )
+    .properties(width=200, height=160, title=alt.Title("Daily Traffic Pattern", fontSize=16))
+)
+
+# Build mosaic layout using Altair's concatenation
+# Layout pattern: "AAB"
+#                 "CDD"
+#                 "EFD"
+
+# Top row: time series (wide) + gauge
+top_row = alt.hconcat(chart_a, chart_b, spacing=20)
+
+# Middle row: bars + scatter (scatter spans into bottom row via vconcat)
+# Bottom row: categories + area (scatter continues from middle)
+
+# Left column for middle and bottom rows
+left_middle = chart_c
+left_bottom = alt.hconcat(chart_e, chart_f, spacing=15)
+left_column = alt.vconcat(left_middle, left_bottom, spacing=15)
+
+# Right side is the tall scatter plot
+middle_bottom_row = alt.hconcat(left_column, chart_d, spacing=20)
+
+# Combine all rows
+mosaic = (
+    alt.vconcat(top_row, middle_bottom_row, spacing=25)
+    .properties(title=alt.Title("subplot-mosaic · altair · pyplots.ai", fontSize=28, anchor="middle", offset=20))
+    .configure(background="white")
+    .configure_view(strokeWidth=0)
+)
+
+# Save outputs
+mosaic.save("plot.png", scale_factor=3.0)
+mosaic.save("plot.html")
diff --git a/plots/subplot-mosaic/metadata/altair.yaml b/plots/subplot-mosaic/metadata/altair.yaml
new file mode 100644
index 0000000000..3e6c21dcf5
--- /dev/null
+++ b/plots/subplot-mosaic/metadata/altair.yaml
@@ -0,0 +1,27 @@
+library: altair
+specification_id: subplot-mosaic
+created: '2025-12-31T10:56:12Z'
+updated: '2025-12-31T11:07:48Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20617510407
+issue: 3002
+python_version: 3.13.11
+library_version: 6.0.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/subplot-mosaic/altair/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/subplot-mosaic/altair/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/subplot-mosaic/altair/plot.html
+quality_score: 91
+review:
+  strengths:
+  - Excellent demonstration of mosaic layout concept using Altair concatenation operators
+    (hconcat, vconcat)
+  - Clean, professional visual design with consistent color scheme (#306998 blue theme)
+  - Good variety of chart types (line, bar, scatter, area, gauge) showing layout flexibility
+  - Descriptive axis labels with appropriate units throughout
+  - Realistic business dashboard scenario with plausible data
+  weaknesses:
+  - No demonstration of empty cells using placeholder characters (e.g., "." for gaps)
+    as mentioned in spec
+  - Scatter plot lacks legend for the color encoding (efficiency gradient)
+  - HTML version could benefit from interactive features (tooltips, selection) which
+    Altair excels at