feat(plotly): implement stem-basic (#5630)

## Implementation: `stem-basic` - python/plotly

Implements the **python/plotly** version of `stem-basic`.

**File:** `plots/stem-basic/implementations/python/plotly.py`

**Parent Issue:** #972

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/25171946708)*

---------

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

Author: github-actions[bot]

plots/stem-basic/implementations/python/plotly.py

@@ -1,82 +1,94 @@
-""" pyplots.ai
+""" anyplot.ai
 stem-basic: Basic Stem Plot
-Library: plotly 6.5.0 | Python 3.13.11
-Quality: 93/100 | Created: 2025-12-23
+Library: plotly 6.7.0 | Python 3.13.13
+Quality: 90/100 | Updated: 2026-04-30
 """
 
+import os
+
 import numpy as np
 import plotly.graph_objects as go
 
 
+# Theme tokens
+THEME = os.getenv("ANYPLOT_THEME", "light")
+PAGE_BG = "#FAF8F1" if THEME == "light" else "#1A1A17"
+INK = "#1A1A17" if THEME == "light" else "#F0EFE8"
+INK_SOFT = "#4A4A44" if THEME == "light" else "#B8B7B0"
+GRID = "rgba(26,26,23,0.20)" if THEME == "light" else "rgba(240,239,232,0.20)"
+BRAND = "#009E73"  # Okabe-Ito position 1
+
 # Data - Discrete signal samples (damped oscillation)
 np.random.seed(42)
 x = np.arange(0, 30)
 y = np.exp(-x / 10) * np.cos(x * 0.8) + np.random.randn(30) * 0.05
 
-# Create figure
+# Plot
 fig = go.Figure()
 
-# Add baseline at y=0
+# Baseline at y=0
 fig.add_trace(
     go.Scatter(
         x=[x.min() - 0.5, x.max() + 0.5],
         y=[0, 0],
         mode="lines",
-        line={"color": "#333333", "width": 2},
+        line={"color": INK_SOFT, "width": 2},
         showlegend=False,
         hoverinfo="skip",
     )
 )
 
-# Add stems (vertical lines from baseline to data points)
+# Stems (vertical lines from baseline to data points)
 for xi, yi in zip(x, y, strict=True):
     fig.add_trace(
         go.Scatter(
-            x=[xi, xi],
-            y=[0, yi],
-            mode="lines",
-            line={"color": "#306998", "width": 2},
-            showlegend=False,
-            hoverinfo="skip",
+            x=[xi, xi], y=[0, yi], mode="lines", line={"color": BRAND, "width": 2}, showlegend=False, hoverinfo="skip"
         )
     )
 
-# Add markers at the top of each stem
+# Markers at the top of each stem
 fig.add_trace(
     go.Scatter(
         x=x,
         y=y,
         mode="markers",
-        marker={"color": "#306998", "size": 16, "line": {"color": "white", "width": 2}},
+        marker={"color": BRAND, "size": 16, "line": {"color": PAGE_BG, "width": 2}},
         showlegend=False,
         hovertemplate="Sample: %{x}<br>Amplitude: %{y:.3f}<extra></extra>",
     )
 )
 
-# Update layout for 4800x2700 px
+# Style
 fig.update_layout(
-    title={"text": "stem-basic · plotly · pyplots.ai", "font": {"size": 42}, "x": 0.5, "xanchor": "center"},
+    paper_bgcolor=PAGE_BG,
+    plot_bgcolor=PAGE_BG,
+    font={"color": INK},
+    title={
+        "text": "stem-basic · plotly · anyplot.ai",
+        "font": {"size": 42, "color": INK},
+        "x": 0.5,
+        "xanchor": "center",
+    },
     xaxis={
-        "title": {"text": "Sample Index", "font": {"size": 36}},
-        "tickfont": {"size": 28},
-        "gridcolor": "rgba(0,0,0,0.1)",
+        "title": {"text": "Sample Index (n)", "font": {"size": 36, "color": INK}},
+        "tickfont": {"size": 28, "color": INK_SOFT},
+        "gridcolor": GRID,
         "gridwidth": 1,
         "zeroline": False,
+        "linecolor": INK_SOFT,
     },
     yaxis={
-        "title": {"text": "Amplitude", "font": {"size": 36}},
-        "tickfont": {"size": 28},
-        "gridcolor": "rgba(0,0,0,0.1)",
+        "title": {"text": "Amplitude (a.u.)", "font": {"size": 36, "color": INK}},
+        "tickfont": {"size": 28, "color": INK_SOFT},
+        "gridcolor": GRID,
         "gridwidth": 1,
         "zeroline": False,
+        "linecolor": INK_SOFT,
     },
-    template="plotly_white",
     margin={"l": 120, "r": 60, "t": 130, "b": 110},
     showlegend=False,
 )
 
-# Save as PNG (4800x2700 px)
-fig.write_image("plot.png", width=1600, height=900, scale=3)
-
-# Save interactive HTML
-fig.write_html("plot.html")
+# Save
+fig.write_image(f"plot-{THEME}.png", width=1600, height=900, scale=3)
+fig.write_html(f"plot-{THEME}.html", include_plotlyjs="cdn")