feat(altair): implement errorbar-basic (#9520)

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

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

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

**Parent Issue:** #973

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/anyplot/actions/runs/28473279872)*

---------

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

@@ -1,7 +1,7 @@
 """ anyplot.ai
 errorbar-basic: Basic Error Bar Plot
-Library: altair 6.1.0 | Python 3.14.4
-Quality: 88/100 | Updated: 2026-04-25
+Library: altair 6.2.2 | Python 3.13.14
+Quality: 94/100 | Updated: 2026-06-30
 """
 
 import importlib
@@ -14,22 +14,23 @@
 alt = importlib.import_module("altair")
 np = importlib.import_module("numpy")
 pd = importlib.import_module("pandas")
-
+PILImage = importlib.import_module("PIL.Image")
 
 # Theme tokens
 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"
-BRAND = "#009E73"  # Okabe-Ito position 1
+BRAND = "#009E73"  # Imprint palette position 1
+ACCENT = "#BD8233"  # Imprint ochre — highlights peak-response group
 
 # Data
 np.random.seed(42)
 categories = ["Control", "Treatment A", "Treatment B", "Treatment C", "Treatment D", "Treatment E"]
 y_values = [25.3, 38.7, 42.1, 35.8, 48.2, 31.5]
 
-# Asymmetric errors: Treatment C and D show notably different lower/upper bounds
+# Asymmetric errors: Treatment C shows wide lower bound; Treatment D peaks highest
 asymmetric_lower = [2.1, 3.5, 2.8, 6.5, 4.8, 2.5]
 asymmetric_upper = [2.1, 3.5, 2.8, 2.8, 2.2, 2.5]
 
@@ -48,19 +49,29 @@
 
 base = alt.Chart(df).encode(x=alt.X("category:N", title="Experimental Group", sort=categories))
 
-error_bars = base.mark_rule(strokeWidth=3, color=BRAND).encode(
-    y=alt.Y("error_lower:Q", title=y_title, scale=y_scale), y2="error_upper:Q"
+# Condition-based encoding: Treatment D (peak response) accented in ochre via alt.condition()
+highlight = alt.datum.category == "Treatment D"
+
+error_bars = base.mark_rule(strokeWidth=3).encode(
+    y=alt.Y("error_lower:Q", title=y_title, scale=y_scale),
+    y2="error_upper:Q",
+    color=alt.condition(highlight, alt.value(ACCENT), alt.value(BRAND)),
 )
 
-caps_top = base.mark_tick(thickness=3, size=22, color=BRAND).encode(
-    y=alt.Y("error_upper:Q", title=y_title, scale=y_scale)
+caps_top = base.mark_tick(thickness=3, size=22).encode(
+    y=alt.Y("error_upper:Q", title=y_title, scale=y_scale),
+    color=alt.condition(highlight, alt.value(ACCENT), alt.value(BRAND)),
 )
-caps_bottom = base.mark_tick(thickness=3, size=22, color=BRAND).encode(
-    y=alt.Y("error_lower:Q", title=y_title, scale=y_scale)
+
+caps_bottom = base.mark_tick(thickness=3, size=22).encode(
+    y=alt.Y("error_lower:Q", title=y_title, scale=y_scale),
+    color=alt.condition(highlight, alt.value(ACCENT), alt.value(BRAND)),
 )
 
-points = base.mark_circle(size=320, color=BRAND).encode(
+points = base.mark_circle().encode(
     y=alt.Y("value:Q", title=y_title, scale=y_scale),
+    color=alt.condition(highlight, alt.value(ACCENT), alt.value(BRAND)),
+    size=alt.condition(highlight, alt.value(480), alt.value(280)),
     tooltip=[
         alt.Tooltip("category:N", title="Group"),
         alt.Tooltip("value:Q", title="Mean", format=".2f"),
@@ -69,28 +80,54 @@
     ],
 )
 
+# Annotation surfacing the "Peak response" label for Treatment D
+peak_df = df[df["category"] == "Treatment D"]
+annotation = (
+    alt.Chart(peak_df)
+    .mark_text(dy=-28, color=ACCENT, fontSize=10, fontStyle="italic", text="Peak response")
+    .encode(x=alt.X("category:N", sort=categories), y=alt.Y("error_upper:Q", scale=y_scale))
+)
+
 chart = (
-    alt.layer(error_bars, caps_bottom, caps_top, points)
+    alt.layer(error_bars, caps_bottom, caps_top, points, annotation)
     .properties(
-        width=1600,
-        height=900,
+        width=620,
+        height=320,
         background=PAGE_BG,
-        title=alt.Title("errorbar-basic · altair · anyplot.ai", fontSize=28, color=INK, anchor="start", offset=20),
+        title=alt.Title(
+            "errorbar-basic · python · altair · anyplot.ai", fontSize=16, color=INK, anchor="start", offset=20
+        ),
     )
     .configure_view(fill=PAGE_BG, stroke=None)
     .configure_axis(
-        labelFontSize=18,
-        titleFontSize=22,
+        labelFontSize=11,
+        titleFontSize=12,
         labelColor=INK_SOFT,
         titleColor=INK,
         domainColor=INK_SOFT,
+        domainOpacity=0,
         tickColor=INK_SOFT,
         gridColor=INK,
-        gridOpacity=0.10,
+        gridOpacity=0.13,
         labelAngle=0,
     )
     .configure_legend(fillColor=ELEVATED_BG, strokeColor=INK_SOFT, labelColor=INK_SOFT, titleColor=INK)
 )
 
-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 target — only expand, never crop (AR-09 guard)
+TW, TH = 3200, 1800
+_img = PILImage.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 = PILImage.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")