feat(altair): implement waterfall-basic (#5743)

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

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

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

**Parent Issue:** #777

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/25410774625)*

---------

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

Author: github-actions[bot]

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

@@ -1,24 +1,37 @@
-""" pyplots.ai
+""" anyplot.ai
 waterfall-basic: Basic Waterfall Chart
-Library: altair 6.0.0 | Python 3.13.11
-Quality: 92/100 | Created: 2025-12-24
+Library: altair 6.1.0 | Python 3.13.13
+Quality: 92/100 | Updated: 2026-05-06
 """
 
+import os
+
 import altair as alt
 import pandas as pd
 
 
+# Theme tokens (see prompts/default-style-guide.md)
+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"
+
+# Okabe-Ito palette for waterfall types
+POSITIVE_COLOR = "#009E73"  # Position 1: brand green for positive
+NEGATIVE_COLOR = "#D55E00"  # Position 2: vermillion/red for negative
+TOTAL_COLOR = "#0072B2"  # Position 3: blue for totals
+
 # Data: Quarterly financial breakdown from revenue to net income
 categories = ["Revenue", "Cost of Goods", "Gross Profit", "Operating Expenses", "Other Income", "Taxes", "Net Income"]
-# Values: First and last are totals (None), middle values are changes
 values = [500, -200, None, -150, 25, -45, None]
 
 # Calculate running totals and bar positions
 n = len(categories)
 running_total = [0] * n
 bar_bottom = [0] * n
 bar_top = [0] * n
-bar_types = []  # 'total', 'positive', 'negative'
+bar_types = []
 
 running_total[0] = values[0]
 bar_bottom[0] = 0
@@ -28,13 +41,11 @@
 current = values[0]
 for i in range(1, n):
     if values[i] is None:
-        # Subtotal bar (Gross Profit or Net Income)
         running_total[i] = current
         bar_bottom[i] = 0
         bar_top[i] = current
         bar_types.append("total")
     else:
-        # Change bar
         running_total[i] = current + values[i]
         if values[i] >= 0:
             bar_bottom[i] = current
@@ -46,7 +57,7 @@
             bar_types.append("negative")
         current = running_total[i]
 
-# Create display values for labels (show change for non-totals, running total for totals)
+# Create display values for labels
 display_values = []
 for i, val in enumerate(values):
     if val is None:
@@ -70,16 +81,16 @@
     }
 )
 
-# Color scale: Python Blue for totals, green for positive, red for negative
-color_scale = alt.Scale(domain=["total", "positive", "negative"], range=["#306998", "#4CAF50", "#E53935"])
+# Color scale using Okabe-Ito palette
+color_scale = alt.Scale(domain=["total", "positive", "negative"], range=[TOTAL_COLOR, POSITIVE_COLOR, NEGATIVE_COLOR])
 
 # Sort by order field
 sort_order = alt.EncodingSortField(field="order", order="ascending")
 
 # Create bar chart using bar marks with y and y2
 bars = (
     alt.Chart(df)
-    .mark_bar(size=65, stroke="white", strokeWidth=2)
+    .mark_bar(size=65, stroke=INK_SOFT, strokeWidth=2)
     .encode(
         x=alt.X(
             "category:N",
@@ -96,7 +107,7 @@
 # Value labels on bars
 labels = (
     alt.Chart(df)
-    .mark_text(fontSize=18, fontWeight="bold", color="white")
+    .mark_text(fontSize=18, fontWeight="bold", color=INK)
     .encode(x=alt.X("category:N", sort=sort_order), y=alt.Y("label_y:Q"), text="display_value:N")
 )
 
@@ -112,18 +123,26 @@
 # Connector lines using rule mark
 connectors = (
     alt.Chart(df_connectors)
-    .mark_rule(color="#666666", strokeDash=[6, 4], strokeWidth=2)
+    .mark_rule(color=INK_SOFT, strokeDash=[6, 4], strokeWidth=2)
     .encode(x=alt.X("x:N", sort=sort_order), x2=alt.X2("x2:N"), y=alt.Y("y:Q"))
 )
 
 # Combine all layers
 chart = (
     alt.layer(connectors, bars, labels)
-    .properties(width=1600, height=900, title=alt.Title("waterfall-basic · altair · pyplots.ai", fontSize=28))
-    .configure_axis(grid=True, gridOpacity=0.3, gridDash=[4, 4])
-    .configure_view(strokeWidth=0)
+    .properties(
+        width=1600,
+        height=900,
+        background=PAGE_BG,
+        title=alt.Title("waterfall-basic · altair · anyplot.ai", fontSize=28, color=INK),
+    )
+    .configure_view(fill=PAGE_BG, stroke=INK_SOFT, continuousWidth=1600, continuousHeight=900)
+    .configure_axis(
+        domainColor=INK_SOFT, tickColor=INK_SOFT, gridColor=INK, gridOpacity=0.10, labelColor=INK_SOFT, titleColor=INK
+    )
+    .configure_title(color=INK)
 )
 
-# Save as PNG and HTML
-chart.save("plot.png", scale_factor=3.0)
-chart.save("plot.html")
+# Save as PNG and HTML with theme suffix
+chart.save(f"plot-{THEME}.png", scale_factor=3.0)
+chart.save(f"plot-{THEME}.html")