Bar and pie diagrams

Diagrams give the quickest understanding of data. They are less accurate than tables (you cannot read exact figures from a picture), but they are more effective: a glance shows the pattern.

Bar diagrams:

• A simple bar diagram has bars of equal width, equally spaced, all starting from zero. Only the height (or length) of a bar shows its value; the width carries no information. So it is called a one-dimensional diagram.
• A multiple bar diagram places two or more bars side by side for each item, to compare sets of data: for example, the female literacy rate of each state in 2001 and in 2011.
• A component bar diagram divides each bar into parts, to show what a total is made of: for example, children in each age group split into those enrolled in school and those out of school.

A pie diagram is a circle divided into slices, each showing a share of the whole. The whole circle is 360°, so 1 per cent is 3.6°. Each slice's angle = its percentage × 3.6. If a family spends 25% of its income on food, that slice is 25 × 3.6 = 90°; a 40% share takes 40 × 3.6 = 144°.

So: use bars to compare values, and a pie to show how a whole is shared.