feat(altair): implement streamline-basic (#2918)

## Implementation: `streamline-basic` - altair

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

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

**Parent Issue:** #2861

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20608708674)*

---------

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

plots/streamline-basic/implementations/altair.py

@@ -0,0 +1,95 @@
+""" pyplots.ai
+streamline-basic: Basic Streamline Plot
+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
+
+
+# Disable data row limit
+alt.data_transformers.disable_max_rows()
+
+# Data - Create a vector field for a vortex flow (u = -y, v = x)
+np.random.seed(42)
+
+# Generate streamlines using Euler integration - flat KISS structure
+streamlines_data = []
+streamline_id = 0
+
+# Starting points at different radii for vortex visualization
+radii = [0.4, 0.7, 1.0, 1.4, 1.8, 2.2, 2.6, 3.0]
+n_per_radius = 6
+dt = 0.03
+max_steps = 250
+
+for r in radii:
+    for i in range(n_per_radius):
+        angle = 2 * np.pi * i / n_per_radius + (r * 0.1)
+        x = r * np.cos(angle)
+        y = r * np.sin(angle)
+        points = [(x, y)]
+
+        # Trace streamline using Euler integration
+        for _ in range(max_steps):
+            # Vector field: circular vortex (u = -y, v = x)
+            u = -y
+            v = x
+            mag = np.sqrt(u**2 + v**2)
+            if mag < 1e-6:
+                break
+            # Normalize and step
+            x_new = x + dt * u / mag
+            y_new = y + dt * v / mag
+            # Stop if out of bounds
+            if abs(x_new) > 3.2 or abs(y_new) > 3.2:
+                break
+            x, y = x_new, y_new
+            points.append((x, y))
+
+        # Only include streamlines with enough points
+        if len(points) > 5:
+            for j, (px, py) in enumerate(points):
+                # Velocity magnitude equals distance from center in this vortex
+                vel = np.sqrt(px**2 + py**2)
+                streamlines_data.append(
+                    {"x": float(px), "y": float(py), "streamline_id": streamline_id, "order": j, "velocity": float(vel)}
+                )
+            streamline_id += 1
+
+df = pd.DataFrame(streamlines_data)
+
+# Compute average velocity per streamline for color encoding
+avg_velocity = df.groupby("streamline_id")["velocity"].mean().reset_index()
+avg_velocity.columns = ["streamline_id", "avg_velocity"]
+df = df.merge(avg_velocity, on="streamline_id")
+
+# Create the streamline chart using line marks
+# Color by average velocity (flow speed) for each streamline
+chart = (
+    alt.Chart(df)
+    .mark_line(strokeWidth=2.5, opacity=0.85)
+    .encode(
+        x=alt.X("x:Q", title="X Position (units)", scale=alt.Scale(domain=[-3.5, 3.5])),
+        y=alt.Y("y:Q", title="Y Position (units)", scale=alt.Scale(domain=[-3.5, 3.5])),
+        color=alt.Color(
+            "avg_velocity:Q",
+            scale=alt.Scale(scheme="viridis"),
+            title="Flow Speed",
+            legend=alt.Legend(titleFontSize=18, labelFontSize=16, gradientLength=200),
+        ),
+        detail="streamline_id:N",
+        order="order:O",
+    )
+    .properties(
+        width=1600, height=900, title=alt.Title("streamline-basic · altair · pyplots.ai", fontSize=28, anchor="middle")
+    )
+    .configure_axis(labelFontSize=18, titleFontSize=22, grid=True, gridColor="#cccccc", gridOpacity=0.4)
+    .configure_view(strokeWidth=0)
+)
+
+# Save as PNG and HTML
+chart.save("plot.png", scale_factor=3.0)
+chart.save("plot.html")

plots/streamline-basic/metadata/altair.yaml

@@ -0,0 +1,28 @@
+library: altair
+specification_id: streamline-basic
+created: '2025-12-31T00:18:39Z'
+updated: '2025-12-31T00:28:21Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20608708674
+issue: 2861
+python_version: 3.13.11
+library_version: 6.0.0
+preview_url: https://storage.googleapis.com/pyplots-images/plots/streamline-basic/altair/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/streamline-basic/altair/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/streamline-basic/altair/plot.html
+quality_score: 91
+review:
+  strengths:
+  - Excellent implementation of Euler integration to trace streamlines in a declarative
+    visualization library
+  - Clean viridis color encoding showing velocity magnitude increases with distance
+    from center
+  - Proper use of detail encoding to separate individual streamlines and order encoding
+    for correct line drawing
+  - Well-balanced streamline density with multiple radii covering the full flow field
+  - Title and font sizes meet all readability requirements
+  weaknesses:
+  - Axis labels lack units - should be "X Position (units)" or specify dimensionless
+  - Some inner streamlines appear slightly crowded/overlapping near the center
+  - Does not leverage Altair interactive features (tooltips, zoom/pan) which would
+    enhance flow exploration