feat(altair): implement quiver-basic (#5559)

## Implementation: `quiver-basic` - python/altair

Implements the **python/altair** version of `quiver-basic`.

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

**Parent Issue:** #1014

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/25137092714)*

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Markus Neusinger <2921697+MarkusNeusinger@users.noreply.github.com>

Author: github-actions[bot]

plots/quiver-basic/implementations/python/altair.py

@@ -1,15 +1,24 @@
-""" pyplots.ai
+""" anyplot.ai
 quiver-basic: Basic Quiver Plot
-Library: altair 6.0.0 | Python 3.13.11
-Quality: 91/100 | Created: 2025-12-23
+Library: altair 6.1.0 | Python 3.13.13
+Quality: 83/100 | Updated: 2026-04-29
 """
 
+import os
+
 import altair as alt
 import numpy as np
 import pandas as pd
 
 
-# Data - Create a 15x15 grid with circular rotation pattern (u = -y, v = x)
+# Theme tokens
+THEME = os.getenv("ANYPLOT_THEME", "light")
+PAGE_BG = "#FAF8F1" if THEME == "light" else "#1A1A17"
+ELEVATED_BG = "#FFFDF6" if THEME == "light" else "#242420"
+INK = "#1A1A17" if THEME == "light" else "#F0EFE8"
+INK_SOFT = "#4A4A44" if THEME == "light" else "#B8B7B0"
+
+# Data - 15x15 grid with circular rotation field (u = -y, v = x)
 np.random.seed(42)
 grid_size = 15
 x_range = np.linspace(-2, 2, grid_size)
@@ -22,76 +31,52 @@
 U = -y_flat
 V = x_flat
 
-# Calculate magnitude for color encoding
+# Magnitude for color encoding
 magnitude = np.sqrt(U**2 + V**2)
 
-# Normalize vectors for consistent arrow length, then scale
+# Scale vectors proportionally to magnitude (arrow length encodes magnitude)
 scale = 0.12
-mag_safe = np.where(magnitude > 0, magnitude, 1)
-U_norm = np.where(magnitude > 0, U / mag_safe * scale, 0)
-V_norm = np.where(magnitude > 0, V / mag_safe * scale, 0)
+U_scaled = U * scale
+V_scaled = V * scale
+
+# Arrow tip positions
+x2 = x_flat + U_scaled
+y2 = y_flat + V_scaled
 
-# Create dataframe with arrow start and end points
-df = pd.DataFrame({"x": x_flat, "y": y_flat, "x2": x_flat + U_norm, "y2": y_flat + V_norm, "magnitude": magnitude})
+# Arrowhead geometry — size proportional to arrow length for visual coherence
+angle = np.arctan2(V_scaled, U_scaled)
+arrow_length = magnitude * scale
+head_size = np.maximum(arrow_length * 0.30, 0.005)
 
-# Create arrowhead geometry (small triangle at the end of each arrow)
-arrow_head_size = 0.04
-angle = np.arctan2(V_norm, U_norm)
+n = len(x_flat)
+ids = np.arange(n)
 
-# Left and right points of arrowhead
-df_heads = pd.DataFrame(
+# Vectorized construction of shaft + two arrowhead lines per arrow
+shaft = pd.DataFrame({"x": x_flat, "y": y_flat, "x2": x2, "y2": y2, "magnitude": magnitude, "arrow_id": ids})
+left = pd.DataFrame(
     {
-        "x": df["x2"],
-        "y": df["y2"],
-        "x_left": df["x2"] - arrow_head_size * np.cos(angle - 0.4),
-        "y_left": df["y2"] - arrow_head_size * np.sin(angle - 0.4),
-        "x_right": df["x2"] - arrow_head_size * np.cos(angle + 0.4),
-        "y_right": df["y2"] - arrow_head_size * np.sin(angle + 0.4),
+        "x": x2,
+        "y": y2,
+        "x2": x2 - head_size * np.cos(angle - 0.4),
+        "y2": y2 - head_size * np.sin(angle - 0.4),
         "magnitude": magnitude,
+        "arrow_id": ids,
+    }
+)
+right = pd.DataFrame(
+    {
+        "x": x2,
+        "y": y2,
+        "x2": x2 - head_size * np.cos(angle + 0.4),
+        "y2": y2 - head_size * np.sin(angle + 0.4),
+        "magnitude": magnitude,
+        "arrow_id": ids,
     }
 )
 
-# Build arrow data for line marks (shaft + two head lines per arrow)
-arrow_data = []
-for i in range(len(df)):
-    mag = df.iloc[i]["magnitude"]
-    # Arrow shaft
-    arrow_data.append(
-        {
-            "x": df.iloc[i]["x"],
-            "y": df.iloc[i]["y"],
-            "x2": df.iloc[i]["x2"],
-            "y2": df.iloc[i]["y2"],
-            "magnitude": mag,
-            "arrow_id": i,
-        }
-    )
-    # Left head line
-    arrow_data.append(
-        {
-            "x": df_heads.iloc[i]["x"],
-            "y": df_heads.iloc[i]["y"],
-            "x2": df_heads.iloc[i]["x_left"],
-            "y2": df_heads.iloc[i]["y_left"],
-            "magnitude": mag,
-            "arrow_id": i,
-        }
-    )
-    # Right head line
-    arrow_data.append(
-        {
-            "x": df_heads.iloc[i]["x"],
-            "y": df_heads.iloc[i]["y"],
-            "x2": df_heads.iloc[i]["x_right"],
-            "y2": df_heads.iloc[i]["y_right"],
-            "magnitude": mag,
-            "arrow_id": i,
-        }
-    )
-
-arrow_df = pd.DataFrame(arrow_data)
+arrow_df = pd.concat([shaft, left, right], ignore_index=True)
 
-# Create the chart using rule marks for arrows with magnitude-based coloring
+# Chart — rule marks encode vectors; viridis colormap encodes magnitude
 chart = (
     alt.Chart(arrow_df)
     .mark_rule(strokeWidth=2.5)
@@ -108,12 +93,25 @@
         ),
     )
     .properties(
-        width=1600, height=900, title=alt.Title("quiver-basic · altair · pyplots.ai", fontSize=28, anchor="middle")
+        width=1600,
+        height=900,
+        background=PAGE_BG,
+        title=alt.Title("quiver-basic · altair · anyplot.ai", fontSize=28, anchor="middle"),
+    )
+    .configure_view(fill=PAGE_BG, stroke=INK_SOFT)
+    .configure_axis(
+        labelFontSize=18,
+        titleFontSize=22,
+        labelColor=INK_SOFT,
+        titleColor=INK,
+        domainColor=INK_SOFT,
+        tickColor=INK_SOFT,
+        gridColor=INK,
+        gridOpacity=0.10,
     )
-    .configure_axis(labelFontSize=18, titleFontSize=22, gridOpacity=0.3)
-    .configure_view(strokeWidth=0)
+    .configure_title(color=INK)
+    .configure_legend(fillColor=ELEVATED_BG, strokeColor=INK_SOFT, labelColor=INK_SOFT, titleColor=INK)
 )
 
-# Save as PNG and HTML
-chart.save("plot.png", scale_factor=3.0)
-chart.save("plot.html")
+chart.save(f"plot-{THEME}.png", scale_factor=3.0)
+chart.save(f"plot-{THEME}.html")