Statistical Graphs, Charts and Maps in Geography
Senior 5 · Geography
Statistical Graphs, Charts and Maps in Geography
Lesson 2 of 4
25 min
A line graph follows one quantity through time. Very often a geographer wants something else: to compare separate items at one moment (five provinces, ten crops, six countries). Those items are categories, not points on a continuous scale, and they are drawn as bars. The length of each bar is proportional to its value, so the eye compares lengths, which it does accurately.
This lesson uses the population of Rwanda's five provinces at the 2012 and 2022 censuses (NISR), rounded to two decimal places of a million.
| Province | 2012 (millions) | 2022 (millions) |
|---|---|---|
| Eastern | 2.60 | 3.56 |
| Southern | 2.59 | 3.00 |
| Western | 2.47 | 2.90 |
| Northern | 1.73 | 2.04 |
| Kigali City | 1.13 | 1.75 |
| Rwanda | 10.52 | 13.25 |
Definition
Bar graph: a graph in which each category is shown by a bar (a rectangle) of equal width, whose length is proportional to its value on a scale. Bars may stand up from the x-axis (vertical bars, or columns) or run along from the y-axis (horizontal bars). Example: Figure 4.
A simple bar graph shows one value per category; a group (multiple) bar graph shows two or more values side by side for each category; a divergence bar graph draws bars on both sides of a reference line.
The bars are ranked from largest to smallest, which makes the order obvious: the Eastern Province had the largest population in 2022, and Kigali City the smallest. Ranking is a choice; an alphabetical or west-to-east order is also acceptable if the question asks for it. Every bar has the same width, and the gaps between them are equal, because width means nothing here: only length carries the value.
Worked example 1: scale and bar length
Problem. You draw Figure 4 with a vertical scale of 1 cm to 0.5 million. How tall is the bar for the Northern Province, and what is the tallest bar?
Solution. Northern: . The tallest bar is the Eastern Province: . Check that 7.1 cm fits on the paper before you start; if not, halve the scale to 1 cm to 1 million.
Putting the 2012 and 2022 bars side by side for each province shows two things at once: the size of each province, and how much it grew. Every province grew, but the gap between the pair of bars is far wider for the Eastern Province and Kigali City than for the other three. In 2012 the Eastern and Southern Provinces were almost equal (2.60 and 2.59 million); by 2022 the Eastern Province was more than half a million larger.
Worked example 2: percentage change
Problem. Calculate the percentage growth of Kigali City from 2012 to 2022, using the full census totals of 1,132,686 and 1,745,555.
Solution.
Kigali City grew by 54.1% in ten years. The national figure, from 10,515,973 to 13,246,394, is 26.0%. So Kigali grew about twice as fast as Rwanda as a whole: evidence of migration to the city as well as natural increase.
Figure 6 subtracts the national growth (26.0%) from each province's growth. Bars to the right grew faster than the country; bars to the left grew more slowly. The pattern is now unmistakable: Kigali City and the Eastern Province gained population faster than the national rate, while the Northern, Western and Southern Provinces all lagged by 8 to 10 percentage points. A geographer would go on to ask why: the capital's jobs and services, and the Eastern Province's lower density and room for new settlement, are possible explanations to test with migration data.
Definition
Divergence bar graph: a bar graph in which bars extend in opposite directions from a central reference line (zero, a mean or a national value), usually coloured differently on each side. Example: Figure 6, growth above or below the national rate. It is the bar form of the divergence line graph and is used for categories rather than a time series.
| Type | Best for | Advantages | Disadvantages |
|---|---|---|---|
| Simple | One value for each of several categories | Easy to draw and read; values can be read off accurately | Shows one variable only; many categories make it crowded |
| Group | Two or three values per category (years, sexes, places) | Compares within and between categories at once | More than three bars per group becomes hard to read; needs a key |
| Divergence | Values above or below a reference (gains and losses, anomalies) | Shows at once which items are above or below and by how much | Hides absolute values; each bar needs a calculation |
Horizontal bars are better when category names are long (district or country names), because the names can be written across the page. A related form, the divided (compound) bar, splits one bar into parts of a whole; it belongs with pie charts and is treated in the next lesson.
Warning
Common mistakes
Tip
Exam technique. When asked to "compare" with a group bar graph, write sentences that contain both bars: "The Eastern Province grew from 2.60 million to 3.56 million, the largest absolute increase (0.97 million), while the Northern Province grew by only 0.31 million." A comparison needs two figures and a link word (while, whereas, but).
Practice
Summary
Sources and credits
Written by Ganzaa. Provincial and national population totals: National Institute of Statistics of Rwanda (NISR), statistics.gov.rw, 2012 and 2022 Population and Housing Census results. Growth rates and differences were calculated by Ganzaa from those totals. All figures were drawn by Ganzaa.
