feat(letsplot): implement step-basic (#5627)

## Implementation: `step-basic` - python/letsplot

Implements the **python/letsplot** version of `step-basic`.

**File:** `plots/step-basic/implementations/python/letsplot.py`

**Parent Issue:** #956

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/25166044399)*

---------

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/step-basic/implementations/python/letsplot.py

@@ -1,47 +1,84 @@
-""" pyplots.ai
+""" anyplot.ai
 step-basic: Basic Step Plot
-Library: letsplot 4.8.2 | Python 3.13.11
-Quality: 92/100 | Created: 2025-12-23
+Library: letsplot 4.9.0 | Python 3.13.13
+Quality: 88/100 | Updated: 2026-04-30
 """
 
+import os
+
 import numpy as np
 import pandas as pd
-from lets_plot import *  # noqa: F403
-from lets_plot.export import ggsave as export_ggsave
+from lets_plot import (
+    LetsPlot,
+    aes,
+    element_blank,
+    element_line,
+    element_rect,
+    element_text,
+    geom_area,
+    geom_point,
+    geom_step,
+    geom_text,
+    ggplot,
+    ggsave,
+    ggsize,
+    labs,
+    scale_x_continuous,
+    theme,
+    theme_minimal,
+)
 
 
-LetsPlot.setup_html()  # noqa: F405
+LetsPlot.setup_html()
+
+# 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"
+RULE = "rgba(26,26,23,0.10)" if THEME == "light" else "rgba(240,239,232,0.10)"
+BRAND = "#009E73"  # Okabe-Ito position 1 — always first series
 
 # Data - Monthly cumulative sales figures (in thousands)
 np.random.seed(42)
 months = np.arange(1, 13)
-# Monthly sales that accumulate over the year
 monthly_sales = np.array([45, 52, 48, 61, 55, 72, 68, 75, 82, 78, 91, 95])
 cumulative_sales = np.cumsum(monthly_sales)
 
 df = pd.DataFrame({"month": months, "cumulative_sales": cumulative_sales})
 
-# Plot - Step plot showing cumulative values that remain constant between changes
+# Annotation: label the year-end total near the final data point
+total = int(cumulative_sales[-1])
+label_df = pd.DataFrame({"month": [11.4], "cumulative_sales": [total + 52], "label": [f"Year-end total: ${total}K"]})
+
+# Plot
 plot = (
-    ggplot(df, aes(x="month", y="cumulative_sales"))  # noqa: F405
-    + geom_step(color="#306998", size=2, direction="hv")  # noqa: F405
-    + geom_point(color="#FFD43B", size=5, stroke=1)  # noqa: F405
-    + labs(  # noqa: F405
-        x="Month", y="Cumulative Sales ($K)", title="step-basic · letsplot · pyplots.ai"
+    ggplot(df, aes(x="month", y="cumulative_sales"))
+    + geom_area(fill=BRAND, alpha=0.15)
+    + geom_step(color=BRAND, size=2, direction="hv")
+    + geom_point(color=BRAND, size=6, alpha=0.9)
+    + geom_text(
+        data=label_df, mapping=aes(x="month", y="cumulative_sales", label="label"), color=INK_SOFT, size=13, hjust=1
     )
-    + scale_x_continuous(breaks=list(range(1, 13)))  # noqa: F405
-    + ggsize(1600, 900)  # noqa: F405
-    + theme_minimal()  # noqa: F405
-    + theme(  # noqa: F405
-        axis_text=element_text(size=16),  # noqa: F405
-        axis_title=element_text(size=20),  # noqa: F405
-        plot_title=element_text(size=24),  # noqa: F405
-        panel_grid=element_line(color="#CCCCCC", size=0.5, linetype="dashed"),  # noqa: F405
+    + labs(x="Month", y="Cumulative Sales ($K)", title="step-basic · letsplot · anyplot.ai")
+    + scale_x_continuous(breaks=list(range(1, 13)))
+    + ggsize(1600, 900)
+    + theme_minimal()
+    + theme(
+        plot_background=element_rect(fill=PAGE_BG, color=PAGE_BG),
+        panel_background=element_rect(fill=PAGE_BG),
+        panel_grid_major_x=element_blank(),
+        panel_grid_major_y=element_line(color=RULE, size=0.3),
+        panel_grid_minor=element_blank(),
+        axis_title=element_text(color=INK, size=20),
+        axis_text=element_text(color=INK_SOFT, size=16),
+        axis_line=element_line(color=INK_SOFT),
+        plot_title=element_text(color=INK, size=24),
     )
 )
 
 # Save PNG (scale 3x to get 4800 x 2700 px)
-export_ggsave(plot, filename="plot.png", path=".", scale=3)
+ggsave(plot, f"plot-{THEME}.png", path=".", scale=3)
 
 # Save HTML for interactive version
-export_ggsave(plot, filename="plot.html", path=".")
+ggsave(plot, f"plot-{THEME}.html", path=".")