Reading graph scales — Data, 7–10 years
A graph's scale tells us what the spaces and marks stand for. Reading it carefully prevents small differences from looking huge or large differences from disappearing.
Every mark has a value
On a graph, the scale is the number pattern along an axis. If the labels are 0, 5, 10, 15, each equal step means 5, not 1. A bar or point must be read against those values, including any smaller marks between the labelled ones.
Why check the scale?
A picture can make a difference look bigger than it is. This happens when an axis starts above zero or uses very large spaces between labels. The scale solves the problem of showing many values in a small space, but only if we read the numbers before judging the picture.
Read the bar height
A chart has labels 0, 10, 20 and 30, with one small mark halfway between each label. The small mark means 5 because each interval of 10 is split into two. A bar reaching the second small mark above 10 is therefore 20, not 12 or 15. Say the scale aloud before reading the bar.
The trap: counting spaces as ones
Someone may count four spaces and call a bar 4, even when the labels show 0, 20, 40 and 60. That mistake is reasonable because the spaces are easy to see and the numbers are farther apart. But each space is worth 20, so four spaces reach 80 if the scale continues.
Where this helps
You meet graph scales in weather apps, sports results and news reports. A temperature graph may use one degree per mark, while a yearly rainfall graph may use 100 millimetres per mark. Reading the scale keeps the picture honest when you compare real changes.
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