Diagrammatic Presentation — Bar Diagrams

A diagram presents statistical data visually, so that a comparison that would take real effort to see in a table becomes obvious at a glance. Diagrams are drawn from data that has already been classified and tabulated — they do not replace a table, they make it easier to read.

A bar diagram represents each value of a variable by a bar (rectangle) of uniform width, whose length (height) is proportional to the value it represents. Bars are drawn on a common base line with equal spacing between them.

Types of bar diagrams

• Simple bar diagram — one bar per item, used for a single variable across categories (e.g., annual turnover of five Gujarat textile firms).
• Multiple bar diagram — two or more bars grouped together per category, used to compare more than one variable side by side (e.g., a firm's sales in two different years, shown as a pair of bars for each firm).
• Sub-divided (component) bar diagram — a single bar per category is divided into segments, each segment's length proportional to the value of one component; used when the components together make up a meaningful total (e.g., a household's expenditure bar split into food, rent, education, and other).
• Percentage bar diagram — a component bar diagram where every bar is scaled to the same total length (100), so that only the relative share of each component is compared, not the absolute size.

Rules for drawing a bar diagram: all bars must have the same width; the gap between bars must be uniform and roughly half the bar's width; bars should be arranged in a logical order (chronological, or by descending size); the vertical axis must start from zero, otherwise the comparison of lengths is misleading; and every diagram needs a title, and the axes need clear labels with units. …