fix(pygal): address review feedback for line-retention-cohort

Attempt 1/3 - fixes based on AI review

Author: github-actions[bot]

plots/line-retention-cohort/implementations/pygal.py

@@ -1,4 +1,4 @@
-""" pyplots.ai
+"""pyplots.ai
 line-retention-cohort: User Retention Curve by Cohort
 Library: pygal 3.1.0 | Python 3.14.3
 Quality: 80/100 | Created: 2026-03-16
@@ -31,14 +31,23 @@
         retention.append(max(round(prev - max(drop, 0.5), 1), 5.0))
     retention_data[cohort] = retention
 
-# Style
+# Style - reference line color first, then cohort colors fading from muted to vivid
+colors_with_opacity = (
+    "rgba(180, 60, 60, 0.6)",
+    "rgba(48, 105, 152, 0.45)",
+    "rgba(232, 119, 93, 0.55)",
+    "rgba(80, 168, 110, 0.70)",
+    "rgba(212, 168, 67, 0.85)",
+    "rgba(139, 107, 174, 1.0)",
+)
+
 custom_style = Style(
     background="white",
     plot_background="white",
     foreground="#333",
     foreground_strong="#333",
     foreground_subtle="#cccccc",
-    colors=("#306998", "#E8775D", "#50A86E", "#D4A843", "#8B6BAE"),
+    colors=colors_with_opacity,
     title_font_size=72,
     label_font_size=48,
     major_label_font_size=42,
@@ -50,26 +59,48 @@
 chart = pygal.Line(
     width=4800,
     height=2700,
-    title="line-retention-cohort · pygal · pyplots.ai",
+    title="line-retention-cohort \u00b7 pygal \u00b7 pyplots.ai",
     x_title="Weeks Since Signup",
     y_title="Retained Users (%)",
     style=custom_style,
     show_dots=True,
     dots_size=6,
-    stroke_style={"width": 5},
+    stroke_style={"width": 3},
     show_y_guides=True,
     show_x_guides=False,
     legend_at_bottom=True,
     truncate_legend=-1,
-    range=(0, 100),
+    range=(0, 105),
     x_label_rotation=0,
+    value_formatter=lambda x: f"{x:.0f}%" if x is not None else "",
+    tooltip_fancy_mode=True,
+    interpolate="cubic",
 )
 
 chart.x_labels = [str(w) for w in weeks]
 
-for cohort, params in cohorts.items():
+# Add reference threshold line at 20% retention
+chart.add(
+    "20% Retention Threshold",
+    [20.0] * len(weeks),
+    stroke_style={"width": 2, "dasharray": "12, 8"},
+    show_dots=False,
+    dots_size=0,
+)
+
+# Add cohorts with increasing stroke width for newer cohorts
+stroke_widths = [2, 3, 4, 5, 7]
+dot_sizes = [4, 5, 5, 6, 8]
+cohort_list = list(cohorts.items())
+
+for i, (cohort, params) in enumerate(cohort_list):
     label = f"{cohort} (n={params['size']:,})"
-    chart.add(label, retention_data[cohort])
+    chart.add(
+        label,
+        [{"value": v, "label": f"Week {w}: {v:.1f}% retained"} for w, v in zip(weeks, retention_data[cohort], strict=True)],
+        stroke_style={"width": stroke_widths[i]},
+        dots_size=dot_sizes[i],
+    )
 
 # Save
 chart.render_to_file("plot.html")