feat(seaborn): implement sankey-basic (#5604)

## Implementation: `sankey-basic` - python/seaborn

Implements the **python/seaborn** version of `sankey-basic`.

**File:** `plots/sankey-basic/implementations/python/seaborn.py`

**Parent Issue:** #810

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/25156180870)*

---------

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/sankey-basic/implementations/python/seaborn.py

@@ -1,215 +1,173 @@
-""" pyplots.ai
+""" anyplot.ai
 sankey-basic: Basic Sankey Diagram
-Library: seaborn 0.13.2 | Python 3.13.11
-Quality: 78/100 | Created: 2025-12-23
+Library: seaborn 0.13.2 | Python 3.13.13
+Quality: 88/100 | Updated: 2026-04-30
 """
 
-import matplotlib.patches as patches
+import os
+
+import matplotlib.patches as mpatches
 import matplotlib.pyplot as plt
 import numpy as np
 import pandas as pd
 import seaborn as sns
 
 
-# Set seed for reproducibility
-np.random.seed(42)
-
-# Apply seaborn styling
-sns.set_theme(style="white", context="talk", font_scale=1.2)
-
-# Data - Energy flow from sources to sectors (in TWh)
-flows_data = {
-    "source": ["Coal", "Coal", "Coal", "Gas", "Gas", "Gas", "Nuclear", "Nuclear", "Nuclear"],
-    "target": [
-        "Residential",
-        "Commercial",
-        "Industrial",
-        "Residential",
-        "Commercial",
-        "Industrial",
-        "Residential",
-        "Commercial",
-        "Industrial",
-    ],
-    "value": [15, 12, 33, 20, 18, 22, 15, 15, 15],
-}
-df = pd.DataFrame(flows_data)
-
-# Create figure with seaborn styling
-fig, ax = plt.subplots(figsize=(16, 9))
-
-# Use seaborn color palettes - distinct colors for sources and targets
-source_names = df["source"].unique()
-target_names = df["target"].unique()
-source_palette = sns.color_palette("husl", n_colors=len(source_names))
-target_palette = sns.color_palette("Set2", n_colors=len(target_names))
-source_colors = dict(zip(source_names, source_palette, strict=True))
-target_colors = dict(zip(target_names, target_palette, strict=True))
-
-# Calculate node totals
-sources = df.groupby("source")["value"].sum().sort_values(ascending=False)
-targets = df.groupby("target")["value"].sum().sort_values(ascending=False)
-
-# Node dimensions and positions
-node_width = 0.06
-x_source = 0.12
-x_target = 0.88
-gap = 0.025
-total_height = 0.60  # Reduced to leave room for legends at bottom
-
-# Calculate source node positions (left side)
-total_source = sources.sum()
-source_positions = {}
-y_pos = 0.92
-for source, value in sources.items():
-    height = (value / total_source) * total_height
-    source_positions[source] = {"y": y_pos - height, "height": height}
-    y_pos -= height + gap
-
-# Calculate target node positions (right side)
-total_target = targets.sum()
-target_positions = {}
-y_pos = 0.92
-for target, value in targets.items():
-    height = (value / total_target) * total_height
-    target_positions[target] = {"y": y_pos - height, "height": height}
-    y_pos -= height + gap
-
-# Track current position for stacking flows at each node
-source_current_y = {s: source_positions[s]["y"] + source_positions[s]["height"] for s in sources.index}
-target_current_y = {t: target_positions[t]["y"] + target_positions[t]["height"] for t in targets.index}
-
-# Bezier curve parameters
-n_points = 100
-t = np.linspace(0, 1, n_points)
-
-# Sort flows by source then by value for consistent stacking
-df_sorted = df.sort_values(["source", "value"], ascending=[True, False])
-
-# Draw flows with widths proportional to values
+# Theme tokens
+THEME = os.getenv("ANYPLOT_THEME", "light")
+PAGE_BG = "#FAF8F1" if THEME == "light" else "#1A1A17"
+INK = "#1A1A17" if THEME == "light" else "#F0EFE8"
+
+OKABE_ITO = ["#009E73", "#D55E00", "#0072B2", "#CC79A7", "#E69F00", "#56B4E9"]
+
+sns.set_theme(style="white", rc={"figure.facecolor": PAGE_BG, "axes.facecolor": PAGE_BG, "text.color": INK})
+
+# Data — energy flows in TWh (varied magnitudes for clear proportional scaling)
+source_names = ["Gas", "Coal", "Nuclear"]
+target_names = ["Residential", "Industrial", "Commercial"]
+flows = [
+    ("Gas", "Residential", 50),
+    ("Gas", "Industrial", 30),
+    ("Gas", "Commercial", 40),
+    ("Coal", "Industrial", 45),
+    ("Coal", "Residential", 20),
+    ("Coal", "Commercial", 15),
+    ("Nuclear", "Residential", 25),
+    ("Nuclear", "Industrial", 10),
+    ("Nuclear", "Commercial", 10),
+]
+df = pd.DataFrame(flows, columns=["source", "target", "value"])
+
+source_colors = dict(zip(source_names, OKABE_ITO[:3], strict=True))
+target_colors = dict(zip(target_names, OKABE_ITO[3:6], strict=True))
+
+sources = df.groupby("source")["value"].sum().loc[source_names]
+targets = df.groupby("target")["value"].sum().loc[target_names]
+
+# Layout
+NODE_W = 0.055
+X_LEFT, X_RIGHT = 0.13, 0.87
+GAP = 0.022
+TOTAL_H = 0.72
+Y_START = 0.85
+
+source_pos = {}
+y = Y_START
+for name in source_names:
+    h = (sources[name] / sources.sum()) * TOTAL_H
+    source_pos[name] = {"y": y - h, "h": h}
+    y -= h + GAP
+
+target_pos = {}
+y = Y_START
+for name in target_names:
+    h = (targets[name] / targets.sum()) * TOTAL_H
+    target_pos[name] = {"y": y - h, "h": h}
+    y -= h + GAP
+
+src_y = {n: source_pos[n]["y"] + source_pos[n]["h"] for n in source_names}
+tgt_y = {n: target_pos[n]["y"] + target_pos[n]["h"] for n in target_names}
+
+# Figure
+fig, ax = plt.subplots(figsize=(16, 9), facecolor=PAGE_BG)
+ax.set_facecolor(PAGE_BG)
+
+t = np.linspace(0, 1, 120)
+s = t**2 * (3 - 2 * t)  # smoothstep: zero tangents at both endpoints
+
+# Sort flows by source order then target order to minimise crossings
+src_ord = {n: i for i, n in enumerate(source_names)}
+tgt_ord = {n: i for i, n in enumerate(target_names)}
+df["_si"] = df["source"].map(src_ord)
+df["_ti"] = df["target"].map(tgt_ord)
+df_sorted = df.sort_values(["_si", "_ti"])
+
+# Draw flows
 for _, row in df_sorted.iterrows():
-    source = row["source"]
-    target = row["target"]
-    value = row["value"]
-    color = source_colors[source]
-
-    # Calculate band height proportional to flow value
-    source_band_height = (value / sources[source]) * source_positions[source]["height"]
-    target_band_height = (value / targets[target]) * target_positions[target]["height"]
-
-    # Source side coordinates
-    y0_top = source_current_y[source]
-    y0_bot = y0_top - source_band_height
-    source_current_y[source] = y0_bot
-
-    # Target side coordinates
-    y1_top = target_current_y[target]
-    y1_bot = y1_top - target_band_height
-    target_current_y[target] = y1_bot
-
-    # Draw the flow band using cubic bezier curves
-    x0 = x_source + node_width
-    x1 = x_target
-    cx0 = x0 + (x1 - x0) * 0.35
-    cx1 = x0 + (x1 - x0) * 0.65
-
-    # Generate bezier curve points for top and bottom edges
-    top_x = (1 - t) ** 3 * x0 + 3 * (1 - t) ** 2 * t * cx0 + 3 * (1 - t) * t**2 * cx1 + t**3 * x1
-    top_y = (1 - t) ** 3 * y0_top + 3 * (1 - t) ** 2 * t * y0_top + 3 * (1 - t) * t**2 * y1_top + t**3 * y1_top
-    bot_y = (1 - t) ** 3 * y0_bot + 3 * (1 - t) ** 2 * t * y0_bot + 3 * (1 - t) * t**2 * y1_bot + t**3 * y1_bot
-
-    # Draw flow band
-    ax.fill_between(top_x, bot_y, top_y, color=color, alpha=0.65, linewidth=0, edgecolor="none")
-
-# Draw source nodes (left) with seaborn colors
-for source in sources.index:
-    pos = source_positions[source]
-    rect = patches.FancyBboxPatch(
-        (x_source, pos["y"]),
-        node_width,
-        pos["height"],
-        boxstyle="round,pad=0.005,rounding_size=0.015",
-        facecolor=source_colors[source],
-        edgecolor="white",
-        linewidth=2.5,
+    src, tgt, val = row["source"], row["target"], row["value"]
+    bh_src = (val / sources[src]) * source_pos[src]["h"]
+    bh_tgt = (val / targets[tgt]) * target_pos[tgt]["h"]
+
+    y0t, y0b = src_y[src], src_y[src] - bh_src
+    src_y[src] = y0b
+    y1t, y1b = tgt_y[tgt], tgt_y[tgt] - bh_tgt
+    tgt_y[tgt] = y1b
+
+    x0, x1 = X_LEFT + NODE_W, X_RIGHT
+    cx0, cx1 = x0 + (x1 - x0) * 0.35, x0 + (x1 - x0) * 0.65
+    xs = (1 - t) ** 3 * x0 + 3 * (1 - t) ** 2 * t * cx0 + 3 * (1 - t) * t**2 * cx1 + t**3 * x1
+
+    # Gas (dominant source) rendered with heavier alpha for visual emphasis
+    flow_alpha = 0.68 if src == "Gas" else 0.44
+    ax.fill_between(
+        xs, y0b + (y1b - y0b) * s, y0t + (y1t - y0t) * s, color=source_colors[src], alpha=flow_alpha, linewidth=0
+    )
+
+# Draw source nodes and labels
+for name in source_names:
+    pos = source_pos[name]
+    ax.add_patch(
+        mpatches.FancyBboxPatch(
+            (X_LEFT, pos["y"]),
+            NODE_W,
+            pos["h"],
+            boxstyle="round,pad=0.005,rounding_size=0.015",
+            facecolor=source_colors[name],
+            edgecolor=PAGE_BG,
+            linewidth=2,
+        )
     )
-    ax.add_patch(rect)
     ax.text(
-        x_source - 0.015,
-        pos["y"] + pos["height"] / 2,
-        f"{source}\n{sources[source]:.0f} TWh",
+        X_LEFT - 0.015,
+        pos["y"] + pos["h"] / 2,
+        f"{name}\n{sources[name]:.0f} TWh",
         ha="right",
         va="center",
-        fontsize=18,
+        fontsize=20,
         fontweight="bold",
-        color="#2d2d2d",
+        color=INK,
     )
 
-# Draw target nodes (right) with distinct colors from Set2 palette
-for target in targets.index:
-    pos = target_positions[target]
-    rect = patches.FancyBboxPatch(
-        (x_target, pos["y"]),
-        node_width,
-        pos["height"],
-        boxstyle="round,pad=0.005,rounding_size=0.015",
-        facecolor=target_colors[target],
-        edgecolor="white",
-        linewidth=2.5,
+# Draw target nodes and labels
+for name in target_names:
+    pos = target_pos[name]
+    ax.add_patch(
+        mpatches.FancyBboxPatch(
+            (X_RIGHT, pos["y"]),
+            NODE_W,
+            pos["h"],
+            boxstyle="round,pad=0.005,rounding_size=0.015",
+            facecolor=target_colors[name],
+            edgecolor=PAGE_BG,
+            linewidth=2,
+        )
     )
-    ax.add_patch(rect)
     ax.text(
-        x_target + node_width + 0.015,
-        pos["y"] + pos["height"] / 2,
-        f"{target}\n{targets[target]:.0f} TWh",
+        X_RIGHT + NODE_W + 0.015,
+        pos["y"] + pos["h"] / 2,
+        f"{name}\n{targets[name]:.0f} TWh",
         ha="left",
         va="center",
-        fontsize=18,
+        fontsize=20,
         fontweight="bold",
-        color="#2d2d2d",
+        color=INK,
     )
 
-# Create legend using simple patches for sources and targets
-source_handles = [
-    patches.Patch(facecolor=source_colors[s], edgecolor="white", linewidth=1.5, label=s) for s in source_names
-]
-target_handles = [
-    patches.Patch(facecolor=target_colors[t], edgecolor="white", linewidth=1.5, label=t) for t in target_names
-]
-
-# Add source legend on the left
-source_legend = ax.legend(
-    handles=source_handles,
-    title="Energy Sources",
-    loc="lower left",
-    bbox_to_anchor=(0.02, 0.02),
-    fontsize=14,
-    title_fontsize=16,
-    frameon=True,
-    fancybox=True,
-    edgecolor="#cccccc",
-)
-
-# Add target legend on the right
-ax.add_artist(source_legend)
-ax.legend(
-    handles=target_handles,
-    title="Sectors",
-    loc="lower right",
-    bbox_to_anchor=(0.98, 0.02),
-    fontsize=14,
-    title_fontsize=16,
-    frameon=True,
-    fancybox=True,
-    edgecolor="#cccccc",
+ax.set_title("sankey-basic · seaborn · anyplot.ai", fontsize=24, fontweight="medium", color=INK, pad=20)
+# Subtitle highlighting key insight: Gas is the dominant source (49% of total)
+ax.text(
+    0.5,
+    0.93,
+    "Gas supplies 49 % of total energy — the dominant source",
+    ha="center",
+    va="center",
+    fontsize=16,
+    color=source_colors["Gas"],
+    fontstyle="italic",
 )
-
-# Set title using the required format
-ax.set_title("sankey-basic · seaborn · pyplots.ai", fontsize=26, fontweight="bold", pad=25)
-
-# Set axis limits and remove decorations
 ax.set_xlim(0, 1)
 ax.set_ylim(0, 1)
 ax.axis("off")
 
-plt.savefig("plot.png", dpi=300, bbox_inches="tight", facecolor="white")
+plt.tight_layout()
+plt.savefig(f"plot-{THEME}.png", dpi=300, bbox_inches="tight", facecolor=PAGE_BG)