feat(altair): implement dumbbell-basic (#9568)

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

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

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

**Parent Issue:** #945

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/28480646536)*

---------

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/dumbbell-basic/implementations/python/altair.py

@@ -1,13 +1,14 @@
 """ anyplot.ai
 dumbbell-basic: Basic Dumbbell Chart
-Library: altair 6.1.0 | Python 3.14.4
-Quality: 89/100 | Updated: 2026-04-26
+Library: altair 6.2.2 | Python 3.13.14
+Quality: 92/100 | Updated: 2026-06-30
 """
 
 import os
 
 import altair as alt
 import pandas as pd
+from PIL import Image
 
 
 # Theme-adaptive chrome
@@ -17,7 +18,7 @@
 INK = "#1A1A17" if THEME == "light" else "#F0EFE8"
 INK_SOFT = "#4A4A44" if THEME == "light" else "#B8B7B0"
 
-# Okabe-Ito palette positions 1 and 2
+# Imprint palette positions 1 and 2
 COLOR_BEFORE = "#009E73"
 COLOR_AFTER = "#C475FD"
 
@@ -41,67 +42,94 @@
     }
 )
 data["difference"] = data["After"] - data["Before"]
-data = data.sort_values("difference", ascending=True)
+data = data.sort_values("difference", ascending=True).reset_index(drop=True)
 
 # Long-form data for the two dot series
 dots_data = pd.melt(
     data, id_vars=["category", "difference"], value_vars=["Before", "After"], var_name="period", value_name="score"
 )
 
-x_scale = alt.Scale(domain=[45, 90])
+title = "Employee Satisfaction · dumbbell-basic · python · altair · anyplot.ai"
+n = len(title)
+title_fontsize = round(16 * (67 / n)) if n > 67 else 16
+
+x_scale = alt.Scale(domain=[45, 92])
 y_sort = alt.EncodingSortField(field="difference", order="ascending")
 
 # Connecting lines (theme-adaptive subtle ink)
 lines = (
     alt.Chart(data)
-    .mark_rule(strokeWidth=3, color=INK_SOFT, opacity=0.55)
+    .mark_rule(strokeWidth=3, color=INK_SOFT, opacity=0.45)
     .encode(y=alt.Y("category:N", sort=y_sort, title=None), x=alt.X("Before:Q", scale=x_scale), x2=alt.X2("After:Q"))
 )
 
 # Dots for Before / After values
 dots = (
     alt.Chart(dots_data)
-    .mark_circle(size=420, opacity=1.0, stroke=PAGE_BG, strokeWidth=2)
+    .mark_circle(size=350, opacity=1.0, stroke=PAGE_BG, strokeWidth=2)
     .encode(
         y=alt.Y("category:N", sort=y_sort, title=None),
         x=alt.X("score:Q", scale=x_scale, title="Employee Satisfaction Score (%)"),
         color=alt.Color(
             "period:N",
             scale=alt.Scale(domain=["Before", "After"], range=[COLOR_BEFORE, COLOR_AFTER]),
-            legend=alt.Legend(title="Policy Change", labelFontSize=16, titleFontSize=18),
+            legend=alt.Legend(title="Policy Change", labelFontSize=10, titleFontSize=12),
         ),
         tooltip=["category:N", "period:N", "score:Q"],
     )
 )
 
+# Difference labels via transform_calculate — shows the gain at a glance
+diff_labels = (
+    alt.Chart(data)
+    .transform_calculate(label="'+' + toString(datum.difference) + ' pts'")
+    .mark_text(align="left", dx=8, fontSize=11, fontWeight="bold", clip=False)
+    .encode(
+        y=alt.Y("category:N", sort=y_sort, title=None),
+        x=alt.X("After:Q", scale=x_scale),
+        text=alt.Text("label:N"),
+        color=alt.value(INK_SOFT),
+    )
+)
+
 chart = (
-    (lines + dots)
+    (lines + dots + diff_labels)
     .properties(
-        width=1600,
-        height=900,
-        title=alt.Title(
-            "Employee Satisfaction · dumbbell-basic · altair · anyplot.ai",
-            fontSize=28,
-            color=INK,
-            anchor="start",
-            offset=20,
-        ),
+        width=576,
+        height=374,
+        title=alt.Title(title, fontSize=title_fontsize, color=INK, anchor="start", offset=16),
         background=PAGE_BG,
     )
     .configure_view(fill=PAGE_BG, stroke=None)
     .configure_axis(
-        labelFontSize=18,
-        titleFontSize=22,
+        labelFontSize=10,
+        titleFontSize=12,
         domainColor=INK_SOFT,
+        domainOpacity=0,
         tickColor=INK_SOFT,
         gridColor=INK,
         gridOpacity=0.10,
         labelColor=INK_SOFT,
         titleColor=INK,
     )
     .configure_title(color=INK)
-    .configure_legend(fillColor=ELEVATED_BG, strokeColor=INK_SOFT, labelColor=INK_SOFT, titleColor=INK, padding=12)
+    .configure_legend(fillColor=ELEVATED_BG, strokeColor=INK_SOFT, labelColor=INK_SOFT, titleColor=INK, padding=10)
 )
 
-chart.save(f"plot-{THEME}.png", scale_factor=3.0)
+chart.save(f"plot-{THEME}.png", scale_factor=4.0)
+
+# Pad to exact 3200×1800 canvas (vl-convert pads outside width/height)
+TW, TH = 3200, 1800
+_img = Image.open(f"plot-{THEME}.png").convert("RGB")
+_w, _h = _img.size
+if _w > TW or _h > TH:
+    raise SystemExit(
+        f"altair vl-convert produced {_w}×{_h}, exceeds target {TW}×{TH}. "
+        f"Shrink chart .properties(width=, height=) values and re-render."
+    )
+if _w < TW or _h < TH:
+    _canvas = Image.new("RGB", (TW, TH), PAGE_BG)
+    _canvas.paste(_img, ((TW - _w) // 2, (TH - _h) // 2))
+    _canvas.save(f"plot-{THEME}.png")
+
 chart.save(f"plot-{THEME}.html")