Figure 53.1 , Market Share by Product

A pie chart encodes quantitative values as arc length and central angle within a circle that represents 100 percent of the total. The viewer's visual system judges proportion by angle , the size of the "pie slice" , which is a weaker perceptual channel than position on a common axis (used by bar charts) but stronger than area alone.

No more than five slices. Human angle discrimination degrades sharply below ~15°. At six or more slices, multiple segments become visually indistinguishable without reading the labels , at which point the chart is doing no perceptual work, and a sorted horizontal bar chart would be faster to read. This implementation enforces the five-slice limit by collapsing excess segments into "Other." Values must sum to a meaningful whole. A pie chart is categorically wrong for data that does not constitute 100 percent of something.

The data structure is a complete, exhaustive composition of five or fewer categories that sum to 100 percent . The message is a single-distribution story: one category dominates, and the chart communicates that dominance at a glance. The reader needs to walk away with one proportion lodged in memory , not to compare seven categories precisely. That is the pie chart's job description.

A 100 percent stacked bar chart would allow more categories, enable multi-period comparison (one bar per time period), and be more accurate for precise proportion reading. It is the correct upgrade when either of these needs arise. A donut chart (togglable above) is perceptually equivalent to a pie chart but opens the centre for a summary statistic , useful when one number (total, selected percent, dominant category) needs emphasis alongside the distribution.

Direct arc labels , percentage values placed on or adjacent to each slice , are strongly preferred over a detached legend. A legend requires the eye to travel off the chart, match a colour swatch, and return. A direct label eliminates that round-trip. The downside is that very small slices (under ~10°) cannot carry a readable label without a leader line , this implementation uses centroid-positioned labels and suppresses them on slices below 5 percent to prevent collision.

Framework , FT Visual Vocabulary FT Visual Vocabulary category: Part-to-whole , "How a single entity is made up of its components." Abela quadrant: Composition (static, few periods). Tufte principle applied: the circle perimeter is data (100 percent of the whole); every arc is data (each category's share). The only non-data ink is the thin white stroke separating slices , necessary for figure, ground separation, not decoration.

Paste this into Claude Code to generate a working version of this chart, plus its data file. The result will not be a perfect replica , the goal is that the reader can run the prompt, get a chart of this type, and read its source. Generate a complete, self-contained pie chart in D3 v7. Two files:

The ideas in this chapter didn't appear from nowhere. Émile Cheysson was a 19th-century French engineer and statistician who pioneered the use of pie charts and proportional area diagrams in public administration , making the form a workhorse of European government statistics decades after Playfair invented it. Émile Cheysson, circa 1880. AI-generated portrait based on a public domain photograph (Wikimedia Commons).

The ideas in this chapter didn't appear from nowhere. Émile Cheysson was a 19th-century French engineer and statistician who pioneered the use of pie charts and proportional area diagrams in public administration , making the form a workhorse of European government statistics decades after Playfair invented it. Émile Cheysson, circa 1880. AI-generated portrait based on a public domain photograph (Wikimedia Commons).