Diagrammatic Presentation — Pie Diagram

A pie diagram (pie chart) presents data as sectors of a circle, where each sector's angle at the centre is proportional to the value it represents. It is the natural choice whenever the components together add up to a meaningful whole (100% of a household's spending, 100% of a firm's total cost) and the reader's main interest is the share of each component, not its absolute size.

Steps to construct a pie diagram

1. Find the total of all the component values.
2. For each component, compute the percentage share: $\text{percentage} = \dfrac{\text{component value}}{\text{total}} \times 100$.
3. Convert each component's value into a degree of angle, since a full circle has $360^{\circ}$: $\text{angle} = \dfrac{\text{component value}}{\text{total}} \times 360^{\circ}$.
4. Draw a circle of a convenient radius; using a protractor, mark off each sector's angle in turn, starting from the 12 o'clock position and moving clockwise, usually largest component first.
5. Label each sector (name and percentage), and shade/colour sectors differently for clarity.
6. Check: the sum of all the sector angles must equal $360^{\circ}$ — this is the built-in arithmetic check that the pie diagram has been constructed correctly. …