Graphs, charts and mapping of data
Pick the graph to fit the data: bars to compare, a pie for shares of a whole, a line for trends over time, a histogram for continuous frequency.
Which chart for which job
| Chart | Best for | Key feature |
|---|---|---|
| Bar chart | Comparing categories or years | Equal-width bars with gaps; length shows value |
| Histogram | Frequency of continuous class intervals | Adjacent bars, no gaps; area shows frequency |
| Pie chart | Parts of one whole | Sector angle = (value ÷ total) × 360° |
| Line chart | Trend over time | Points joined in time order |
| Table | Exact values for look-up | Title, stub (row heads), caption (column heads), body, source |
| Frequency polygon | Shape of a distribution | Joins the mid-points of histogram bar tops |
| Ogive | Cumulative frequency | Less-than and more-than curves meet at the median |
Worked example: pie-chart angle
A college has 1,200 students, of whom 300 study science. What angle does science get in a pie chart?
- Share = 300 ÷ 1,200 = 0.25, that is 25%.
- Angle = 0.25 × 360° = 90°.
90°
Mapping of data
- Choropleth map: areas shaded by value, such as literacy rate by state.
- Dot map: each dot stands for a fixed count, such as 1,000 people.
- Isopleth map: lines join places with equal values, such as isotherms for temperature.
- Flow map: arrows of varying width show movement, such as migration.
- GIS (Geographic Information System) layers many such data sets on one digital map.
Reading a graph without being misled
- Check where the value axis starts; a cut-off axis exaggerates differences.
- Check units and scale, and whether values are absolute numbers or percentages.
- In a pie chart, compare angles, not the visual size of labels.
- Two pies of different totals cannot be compared by angle alone.
How NET asks this: match a chart with its best use, convert a pie-chart angle to a percentage or an amount, or pick the graph that suits continuous frequency data.
Typical MCQ: “A pie-chart sector measures 72°. What share of the total does it show?” 72 ÷ 360 = 0.2, that is 20%.
Typical MCQ: “Which graph has bars with no gaps between them?” The histogram, because its class intervals are continuous. A bar chart has gaps between separate categories.
Pie: 3.6° for every 1%. Histogram bars touch because the data run on without breaks; bar-chart bars stand apart because the categories do.