MISSION · Data and probability
Reading Bar and Line Graphs
What each kind of graph is built to show
LEVEL 1 · EXPLORE
Make the idea move
Change the controls and watch what happens. Follow your curiosity and see what you notice.
Loading the activity…
LEVEL 2 · LEARN
Unlock the explanation
Open the chapters in any order. Try the examples as you go.
The idea in one sentence
A graph turns a list of numbers into a picture, and each kind of picture is built to answer a different kind of question.
Bars compare, lines track change
Bar graphs put separate things side by side so you can compare heights at a glance: favourite fruits, books read by each child, rainfall in four cities. The categories have no natural order — you could rearrange the bars and lose nothing.
Line graphs join points in order to show how one thing changes over time: temperature through a day, a plant’s height over weeks. The line between two points is a claim that the value moved smoothly between them, which is why you can’t use a line graph for favourite fruits. There is no “halfway between apples and bananas”.
Choosing the wrong one isn’t a style error. It makes a claim about the data that isn’t true.
Read the scale before the picture
Every graph has a scale, and it is the first thing to check. If each gridline is worth 5, a bar reaching the fourth line is 20, not 4.
Scales that don’t start at zero are the classic trick. Two bars of 98 and 100 look almost identical on a scale from 0 to 100 — which is honest, because they nearly are. Start the scale at 97 and one bar looks three times the other. The numbers didn’t change; the impression did. Whenever a difference looks dramatic, look at where the axis begins.
Getting answers out
Most graph questions are one of four kinds, and knowing which one you’ve been asked tells you what to do:
- Read one value. Find the bar or point, follow it across to the scale.
- Compare two. Read both, then subtract.
- Find a total. Read every bar and add — the graph won’t show the total itself.
- Describe a trend. Look at the shape, not the individual points. Rising, falling, flat, or up-then-down.
Common mix-ups
- Counting gridlines instead of reading the scale. Four lines up is not 4 unless each line is worth 1.
- Assuming the axis starts at zero. Check.
- Using a line graph for categories, which invents changes that never happened.
- Reading a pictogram symbol as one item. The key often says one picture means 10.
Check yourself
A bar graph of books read has a scale where each gridline is 5 books. Ali’s bar reaches the third line and Beth’s reaches the seventh. How many did each read, and how many more did Beth read?
Ali read 15 and Beth read 35. Beth read 20 more — found by subtracting, not by counting the four gridlines between them.
LEVEL 3 · REFLECT
Add this quest to your trail
When you feel ready, mark this idea as explored. You can always come back and try it again.
EXPLORER STRATEGIES