Line graphs and change

Q: Which graph is best for seeing how something changes?

Line graph — When something changes over time, a line graph is the way to look at it.

Q: What does a steep part of the line mean?

It is changing quickly — A steep slope means a big change happened in a short time.

Q: On the temperature graph it is 12℃ at 6 o'clock and 24℃ at 12 o'clock. How much did it rise in those 6 hours?

12℃ — 24 − 12 = 12, so it rose by 12℃.

Q: Which statement about the space between two dots is correct?

Nobody measured it, so we don't know — Anything other than the measured points is a guess. Don't state it as a fact.

Q: When you compare two line graphs, what has to match?

The scale on the vertical axis — If the scales are different, the very same change can look completely different.

Where this is used

Temperature, sales, body weight, how an illness spreads. Anything that changes over time is easiest to see on a line graph. Is it going up? Going down? Speeding up? Once you can read that, you can act before it is too late.

Key points

A line graph shows change

A bar graph is for comparing things. A line graph is for watching something change. Time goes along the bottom, the number goes up the side, and you join the dots with straight lines.

How the temperature changes in one day

The slope of the line is how fast it changes

The steeper the line, the faster the change. In the graph above, the steepest part is from 6 to 12 o'clock, so the temperature rose fastest during the morning. Reading a slope like this is exactly the idea you will meet again in linear functions in middle school, and in calculus in high school.

You don't really know what happens between the dots

A line graph just joins the dots with straight lines. Nobody measured what happened between 9 and 12 o'clock, so we cannot say the temperature really rose in a straight line. Graphs are useful, but never state something as a fact where nothing was measured.

Two lines together show the difference

Draw this year's numbers next to the same months from last year, and you can see at a glance whether things are growing. The rule when comparing: use the same scale on the vertical axis for both.

Visitors each month (this year)

Check yourself 5questions

Tap an option to check

Q1 Which graph is best for seeing how something changes?

Q2 What does a steep part of the line mean?

Q3 On the temperature graph it is 12℃ at 6 o'clock and 24℃ at 12 o'clock. How much did it rise in those 6 hours?

Q4 Which statement about the space between two dots is correct?

Q5 When you compare two line graphs, what has to match?

Jobs that use this

Every job uses this. The ones below are just where it shows up most.

Weather forecaster $70k

Store manager $55k

Nurse $80k

Weather forecaster$70k

Reads line graphs of temperature and air pressure to predict the weather ahead. A change in the slope is often the sign that the weather is about to turn.

Maths used: Line graphsRate of changeStatistics

Store manager$55k

Looks at the sales line graph every day to judge whether business is growing or slipping. Noticing the moment sales start to fall can decide whether the store survives.

Maths used: Line graphsAveragesPercentages

Nurse$80k

Records temperature and pulse, then reads the shape of the graph to tell whether a patient is getting better or worse. What matters is the change, not any single reading.

Maths used: Line graphsAveragesRecord keeping