Reading a line chart

By the end of this lesson you can read change over time from a line chart, including rises, falls and flat parts.

Why it matters

"Are things getting better or worse?" is one of the most common questions anyone asks of data. Prices, school places, rainfall, sales: a line chart answers it at a glance, if you know how to read it.

The idea in plain words

Think of the height marks some families draw on a door frame, one mark each birthday. Join the marks with a line, and you can see how fast a child grew, and when.

A line chart does the same. Time runs along the bottom, left to right, in order. Each point is the value at one time. The line joins them, so you see the change.

• The line goes up: the value is rising.
• The line goes down: the value is falling.
• The line is flat: the value is not changing.
• The steeper the line, the faster the change.

Line chart: a chart where values in time order are joined by a line, to show how something changes over time.

Trend: the general direction of the line over time: up, down or flat.

Nigeria's population, 1990 to 2024. The line rises all the way.

See it done

Data: country-indicators.csv (real), Nigeria's rows.

Amara's nephew asks her, "Are there more people in Nigeria than when you were a girl?" She opens the country tab to find out.

Question: how did Nigeria's population change from 1990 to 2024?

1. Check the parts. Title: Nigeria's population, 1990 to 2024. Unit: millions of people. Bottom: years, in order. Source: named at the foot.
2. Read the trend. The line goes up from left to right, every year. The trend is up.
3. Read the marked points. 1990: 97.1 million. 2000: 126.4 million. 2010: 166.6 million. 2024: 232.7 million.
4. Work out the change. 232.7 − 97.1 = 135.6 million more people in 2024 than in 1990.
5. Compare. 232.7 is more than twice 97.1 (2 × 97.1 = 194.2). So the population more than doubled.
6. Say it in one sentence. "Nigeria's population rose every year from 1990 to 2024, from 97.1 million to 232.7 million, more than doubling."

Check a point yourself: filter the country tab to Nigeria, find the row for 1990, and read the population column: 97,120,926, which is 97.1 million (rounded).

Not every line goes up. Here is the carbon dioxide (CO₂) given off by the United Kingdom each year, mostly from burning coal, oil and gas.

A line that falls. It wobbles until about 2007, then drops.

Carbon dioxide (CO₂): a gas given off when fuels such as coal, oil and gas are burned. The country data measures it in million tonnes.

Your turn, with help

Read the United Kingdom chart. What was the value in 1990, and in 2024? What is the trend?

Hint: the two amber dots are labelled. For the trend, look at the whole line, not just one bump.

Show the answer

1990: 601.9 million tonnes. 2024: 312.9 million tonnes. The trend is down: after ups and downs until about 2007, it falls to about half (312.9 is about 52 percent of 601.9).

Your turn, alone

1. By how many million tonnes did the United Kingdom's CO₂ fall from 1990 to 2024?
2. In the Nigeria chart, did the population grow more from 1990 to 2000, or from 2010 to 2024?
3. Draw (on paper) a line that is flat, then rises steeply. Write one sentence that describes it.
4. Why do the years along the bottom have to be in order?

Check your answers

1. 601.9 − 312.9 = 289.0 million tonnes.
2. From 2010 to 2024: 232.7 − 166.6 = 66.1 million, compared with 126.4 − 97.1 = 29.3 million. (It is also a longer period, 14 years against 10.)
3. For example: "The value stayed the same for a while, then rose quickly."
4. The line joins one time to the next. If the years were out of order, the line would jump about and show changes that never happened.

Watch out

• Reading one bump as the trend. Look at the whole line first, then the bumps.
• Ignoring the time gaps. Compare like with like: 10 years is not the same as 14 years.
• Forgetting the axis. Check where the side axis starts. A line can look steep just because the axis was cut short (Module 5).

Say it back

1. What does a line chart show?
2. What does a steep line mean?
3. What was Nigeria's population in 2024, in millions?

Show the answers

1. How a value changes over time.
2. A fast change.
3. 232.7 million.

What is next

You have read two kinds of chart. Next, you make one yourself in Google Sheets, from Amara's data.

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Data in this lesson: Global Carbon Budget (2025); UN World Population Prospects (2024), processed by Our World in Data. Licensed under CC BY 4.0 (UN data: CC BY 3.0 IGO). Extract and continent labels by DataMedics.