Statistical Graphs, Charts and Maps in Geography
Senior 5 · Geography
Statistical Graphs, Charts and Maps in Geography
Lesson 1 of 4
25 min
Rwanda has counted its people in five national censuses. The National Institute of Statistics of Rwanda (NISR) published these totals:
| Census year | 1978 | 1991 | 2002 | 2012 | 2022 |
|---|---|---|---|---|---|
| Population | 4,831,527 | 7,157,551 | 8,128,553 | 10,515,973 | 13,246,394 |
Read as a row of numbers, the table tells you that the population grew. Drawn as a graph, it tells you much more at a glance: how fast it grew, when growth slowed, and when it sped up again. That is the work of a statistical graph. Geographers handle large sets of numbers (rainfall, crop output, population, trade), and graphs, charts and statistical maps let them summarise the data, compare places and periods, pick out trends and exceptions, and show results to readers who will never study the raw table. In the A-level examination you are expected both to draw them accurately and to interpret them: describe the pattern, quote figures from the graph, and suggest reasons.
Definition
Statistical graph (or diagram): a drawing that shows numerical data by lengths, positions, areas or shading, so that values can be compared by eye. Example: a bar graph of the population of each province.
Definition
Line graph: a graph in which values are plotted as points against a scale on the vertical axis (the y-axis) and joined by a line, in the order of a continuous quantity on the horizontal axis (the x-axis), usually time. Example: Figure 1, population against census year.
Three kinds are met in geography: the simple line graph (one set of data), the group or multiple line graph (two or more sets on the same axes) and the divergence line graph (values plotted above and below a reference line).
Figure 1 plots the census table. Notice three things about how it is built.
Now interpret it. The line is steep from 1978 to 1991, almost flat from 1991 to 2002, then steep again. The flattening reflects the 1994 Genocide against the Tutsi and the displacement of people in and around that year. After 2002 the line rises steadily again.
Worked example 1: choosing the scales
Problem. You must draw Figure 1 on graph paper 16 cm wide and 12 cm high. Choose scales for both axes.
Solution.
Choose scales that use most of the paper and are easy multiples (1, 2, 4, 5, 10). An awkward scale causes plotting errors.
Worked example 2: reading a rate from the graph
Problem. Use Figure 1 to find the mean annual growth rate of Rwanda's population between 2012 and 2022.
Solution. If a population grows to in years at a steady rate per year, then , so
The population grew by about 2.3% a year, the figure NISR reports for 2012–2022. The same method gives about 1.2% a year for 1991–2002 and about 3.1% a year for 1978–1991: the graph's three slopes, now in numbers.
A group line graph puts two or more data sets on the same axes so that they can be compared directly. Figure 2 compares the mean rainfall of each month at two stations. Both show two wet seasons, peaking in April and in November, and a dry season from June to August. Station P is wetter in every month. The two lines run nearly parallel, which suggests that the same weather systems bring rain to both places, with more at P.
Rules for a group line graph: use the same units for every line; make each line different in both colour and pattern (solid, dashed) so that it can still be read in a black-and-white copy; give a key; and stop at three or four lines, since more become a tangle.
Definition
Divergence line graph: a line graph in which each value is plotted as its difference (its deviation) from a reference value, such as a long-term mean, which is drawn as a horizontal line. Points above the line are higher than the reference; points below are lower. Example: Figure 3.
A climatologist does not ask only "how much rain fell in 2022?" but "was 2022 wetter or drier than normal, and by how much?" Figure 3 answers that. The long-term mean of Station P, 1,150 mm, is the zero line. Each year is plotted as its deviation:
Worked example 3: plotting and reading a deviation
Problem. In 2022 Station P received 990 mm. (a) What deviation is plotted? (b) Which year had the largest surplus, and how much rain fell that year?
Solution. (a) : the point is plotted 160 mm below the zero line. (b) The highest point is 2018, at mm, so 2018 received .
Shading the area above the line in one colour and the area below in another makes wet and dry runs stand out. In Figure 3 two dry years in a row, 2021 and 2022, follow three wet years out of four. Farmers and water planners care about such runs more than about any single year. Divergence graphs are also used for temperature anomalies, for each district's growth rate compared with the national rate, and for a trade balance (exports minus imports).
| Type | Advantages | Disadvantages |
|---|---|---|
| Simple | Easy to draw and read; shows trend and rate of change clearly | Shows one variable only; the line implies values between points that were never measured |
| Group (multiple) | Direct comparison of several places or items on one scale | Lines can cross and tangle; all sets need the same units and a similar range |
| Divergence | Shows at once which values are above or below normal and by how much | Hides the actual values unless the mean is given; needs a calculation for every point |
Warning
Common mistakes
Tip
Exam technique: describing a graph. Use the pattern TEA: the general Trend, an Example with figures read from the graph (with units and years), and any Anomaly or change of trend. "The population rose from 4.8 million in 1978 to 13.2 million in 2022, but growth almost stopped between 1991 and 2002" earns more than "the population increased".
Practice
Summary
Sources and credits
Written by Ganzaa. Census totals: National Institute of Statistics of Rwanda (NISR), statistics.gov.rw, results of the 1978, 1991, 2002, 2012 and 2022 Population and Housing Censuses; the 2012–2022 growth rate of 2.3% is from the 2022 census main indicators report (NISR, 2023). Figures 2 and 3 use illustrative data invented for teaching. All figures were drawn by Ganzaa.
