Line graphs in real life
Line graphs aren't a school invention. They're the most common chart in newspapers, weather apps and sports broadcasts. Once you spot one, the same reading tricks always apply.
Weather forecasts
Open any weather app and you'll see a line graph: temperature over the next 24 hours or rainfall over the week. The line goes up when it's warmer, down when it's colder.
The forecast line is at 18 °C at 2 pm and at 9 °C at midnight. By how many degrees did the temperature drop?
Drop = 18 − 9 = 9 degrees.
Stock prices
Stock-price charts are also line graphs — usually with the day on the bottom and the price on the side. A sudden vertical jump on the line means a big change in price between two days.
You don't have to be a banker to read one. Just remember: up = more, down = less, steep = changed fast.
Growth charts
When you go to the doctor, your height and weight get plotted on a growth curve — a line graph that shows how kids in general grow with age. Your line is compared against the average to see if your growth is typical.
The horizontal axis is your age; the vertical axis is your height or weight. Your line should generally go up (you grow over time). If it suddenly flattens for a long while, that's interesting to a doctor.
Sports performance
A runner's coach might plot their time per kilometre every week. A line going down means they're getting faster (a smaller time is better in racing). A line going up means they're slowing down — bad sign for race week.
Week 1: 5:00 per km. Week 5: 4:30 per km. By how many seconds per km did they improve?
5 min = 300 s. 4 min 30 s = 270 s. Improvement = 30 seconds per km.
Tips for reading
- Check the axes first. What does the bottom one measure? What does the side one measure? In what units?
- Check the scale. A line graph can look "dramatic" just because the scale is squashed. A jump from 100 to 102 looks huge if the y-axis only goes from 99 to 103. Always read the actual numbers, not just the picture.
- Watch for missing zero. Some line graphs start the y-axis above zero to fit the data tightly. That's fine — but the changes look bigger than they really are.
- Trends, not single points. A line graph is best at showing direction, not exact values. For exact reads, look at the data points.
A puzzle
A plant was measured every Monday for 6 weeks. Heights: 5 cm, 7 cm, 10 cm, 12 cm, 15 cm, 14 cm. Plot a line graph in your head. Between which two consecutive weeks did the plant grow the most? Between which two did it shrink?
Differences week-to-week: +2, +3, +2, +3, −1.
- Most growth: weeks 2→3 and 4→5 tie at +3 cm.
- Shrink: weeks 5→6 (the only negative).
Maybe a leaf fell off — that's the kind of question line graphs lead to.