Picking bar or line

By the end of this lesson you can choose a bar chart for comparing groups and a line chart for change over time.

Why it matters

The wrong chart type can quietly tell a false story. Picking the right one takes one question, and makes your work instantly easier to trust.

The idea in plain words

A line is a promise. It says: "between this point and the next, things changed smoothly, in this direction". That promise only makes sense when the points are in a real order, like time: 2023, then 2024. Bars make no such promise. Each bar stands alone.

So ask one question: is the thing along the bottom time, in order?

• Yes (years, months, days in order): use a line.
• No (fruits, countries, payment methods): use bars.

One question decides it: is the bottom axis time, in order?

What about pie charts? Pies show parts of one whole, but people judge angles and slice sizes less accurately than the ends of bars along a shared scale (Cleveland and McGill, 1984, Journal of the American Statistical Association). A bar chart almost always shows the same thing more clearly.

See it done

Data: items sold per fruit at Amara's stall (synthetic), worked out with SUMIF as in lesson 3.5's "Go further" box.

Question: what chart should show items sold per fruit?

1. Ask the question. Is the bottom axis time, in order? It is fruits. No.
2. Choose bars. Each fruit gets its own bar.
3. See what goes wrong with a line. Here are the same six numbers drawn both ways.

Right: separate fruits, separate bars. Orange (103) clearly sold the most items.

Wrong: the line seems to say items "fell" from Orange to Pineapple. Fruits have no order, so that fall never happened.

4. Read the line chart's false story. It seems to say "items sold dropped sharply after Orange, then rose again towards Mango". But there is no "after". The fruits could be listed in any order, and the line's shape would change completely. The numbers are the same; the line invents a story.

Your turn, with help

Bar or line?

1. Nigeria's population each year from 1990 to 2024.
2. The population of 17 countries in 2024.

Hint: for each one, ask: what goes along the bottom? Is it time, in order?

Show the answer

1. Line. Along the bottom: years, in order.
2. Bar. Along the bottom (or side): countries, which have no order.

Your turn, alone

Bar or line? Say why in a few words.

1. Amara's number of sales on each day, Monday to Saturday.
2. Lucía's pupils by travel_mode.
3. The United Kingdom's CO₂ each year since 1990.
4. Which payment method Amara's customers used.
5. In Sheets, open your Lucía chart from lesson 4.4 and change Chart type to Line chart. What false story does the line now tell? Change it back to a column chart.

Check your answers

1. Line: days in order are time. (A bar chart, like the one you made in lesson 4.4, is also acceptable here, because days have a real order. The rule to never break is: no line through groups that have no order.)
2. Bar. Travel modes have no order.
3. Line. Years in order.
4. Bar. Payment methods have no order.
5. The line seems to show travel "rising" or "falling" from one mode to the next, as if Walk, Bus, Bicycle and Car were in an order. They are not.

Watch out

• Lines through categories. Never join fruits, countries or payment methods with a line.
• Years out of order. A line through 2024, 1990, 2010 (unsorted) zig-zags nonsense. Sort by time first.
• Too many lines. Five or six lines on one chart turn into spaghetti. Show the one that matters in a strong colour, and the rest in grey, or make several small charts.

Say it back

1. What one question decides between bar and line?
2. Why is a line through fruits wrong?
3. Which chart for "population of 17 countries in 2024"?

Show the answers

1. Is the thing along the bottom time, in order?
2. A line suggests change from one point to the next, but fruits have no order, so that change is made up.
3. A bar chart.

What is next

That ends Module 4. You can read and make honest charts. But not every chart you meet will be honest, even when nobody means to trick you. Module 5 shows five common traps, and how to spot each in seconds.