Chart Type for Part-to-Whole. Keywords: part-to-whole, pie, donut, percentage, proportion, share. Best Chart: Pie Chart or Donut. Secondary Options: Stacked Bar, Waffle Chart. When to Use: ≤5 categories; one dominant segment vs rest; emphasis on visual proportion over exact values. When NOT to Use: Categories > 5; slice differences < 5% (visually indistinguishable); user needs precise values; accessibility-first context. Data Volume Threshold: Max 6 slices; beyond that switch to stacked bar 100%. Color Guidance: 5–6 max colors. Contrasting palette. Largest slice at 12 o'clock. Always label slices with %. Accessibility: C — Pie charts fail WCAG for colorblind users. Slices rely on color alone. Avoid as primary chart in a11y contexts.. A11y Fallback: Must provide stacked bar alternative + percentage data table as mandatory fallback. Library: Chart.js, Recharts, D3.js. Interactive Level: Hover + Drill.