Why draw graphs and diagrams?
A table has exact numbers but hides the pattern. A graph shows it at a glance. Rules for every graph: a title, labelled axes with units, a suitable scale, a legend (key) and the data source. Choose the type that fits the data.
Line graph and polygraph
A line graph shows how one variable changes over time (years, months). Time goes on the x-axis; the value goes on the y-axis. Plot each point and join them with a line.
A polygraph has two or more lines on the same axes, each in a different colour or style, to compare variables (for example, the growth of population in two cities).
A climograph-style chart can combine a temperature line with rainfall bars on two y-axes.
Bar diagrams: simple, multiple and compound
Bars have equal width and equal gaps. Only the length shows the value. Bars can be vertical or horizontal.
- Simple bar: one bar per item (e.g. rainfall of each month).
- Multiple bar: two or more bars side by side for each item (e.g. literacy of men and women in each year).
- Compound (divided) bar: one bar divided into parts that add up to the total (e.g. rural + urban population).
Pie diagram
A pie diagram is a circle divided into sectors that show the share of each part in the whole.
Angle = (value of part ÷ total) × 360°. If the data are in percent, angle = % × 3.6°.
Steps: find all angles, check they add to 360°, draw a circle, draw the largest angle first from the 12 o'clock line, go clockwise, shade and label each sector.
Flow diagrams and flowcharts
A flow map (flow diagram) shows movement between places: goods, trains, people, traffic. Arrows or bands start at the origin and end at the destination. Their width is drawn to scale (e.g. 1 mm = 10 trains).
A flowchart shows the steps of a process in order, using boxes joined by arrows (e.g. the steps of making a map).
Thematic maps: dot, choropleth and isopleth
Dot map
Each dot has a fixed value (e.g. 1 dot = 10,000 people). Dots are placed where the thing actually is. Good for showing distribution of population, livestock or crops. Number of dots = value ÷ value of one dot.
Choropleth map
Each unit area (district, block, grid square) is shaded by its value, using graded tones: light = low, dark = high. Steps: arrange data, choose 4–6 classes, pick shades, shade each area, add a key. Good for densities and percentages.
Isopleth map
Lines join places with equal value. Examples: isotherms (temperature), isobars (pressure), isohyets (rainfall), contours (height). Values between points are estimated by interpolation. Good for things that change smoothly over space.
Key formulas and definitions
- Pie angle = (part ÷ total) × 360° (or % × 3.6°)
- Dots needed = value ÷ value of one dot
- Flow width = flow ÷ chosen scale
- Iso-lines: iso = equal (isotherm, isobar, isohyet, contour)
Worked examples
1. Which graph would you draw for the population of a town in 1971, 1981, 1991, 2001, 2011, 2021?
A line graph, because it shows change of one value over time.
2. Land use of a village: farms 90 ha, forest 45 ha, houses 27 ha, water 18 ha. Find the pie angles.
Total = 180 ha. Farms 90/180 × 360 = 180°. Forest 45/180 × 360 = 90°. Houses 27/180 × 360 = 54°. Water 18/180 × 360 = 36°. Check: 180 + 90 + 54 + 36 = 360°.
3. In percent: agriculture 45%, industry 25%, services 30%. Find the angles.
45 × 3.6 = 162°, 25 × 3.6 = 90°, 30 × 3.6 = 108°. Total 360°.
4. A district has 2,40,000 people. On a dot map, 1 dot = 20,000 people. How many dots?
2,40,000 ÷ 20,000 = 12 dots.
5. A town's rural population is 60,000 and urban is 40,000. How will you show both in one bar?
Draw a compound bar of total 1,00,000. Divide it: the lower 60% for rural, upper 40% for urban, with different shades and a key.
6. On a flow map the scale is 1 mm = 5 trains. A route has 35 trains a day. How wide should the band be?
35 ÷ 5 = 7 mm.
7. Which map would you use to show rainfall across a large region? Why?
An isopleth map with isohyets, because rainfall changes gradually over space, and lines joining equal rainfall show this smoothly.
Common mistakes
- Leaving gaps between histogram bars or no gaps between bar-diagram bars. Bar diagrams need equal gaps.
- Pie angles that do not add up to 360°. Always check the total.
- Using a choropleth map for raw totals of big and small areas. Choropleth works best for densities or percentages.
- Drawing isopleth lines that cross each other. Lines of different equal values never cross.