Line graphs — change over time

Line graphs — change over time

A line graph is the chart of choice for showing how something changes over time. The horizontal axis is time (hours, days, months, years), and the vertical axis is the quantity you are measuring (temperature, height, count, money). Each point says "at this moment, the value was this much", and the points are joined by straight line segments.

What a line graph is good at

  • Showing change — rising, falling, staying the same.
  • Showing the highest and lowest values across a time range.
  • Comparing two quantities on the same picture (two lines, in two colours).
  • Spotting patterns: an up-then-down shape, a steady climb, a sudden drop.

A line graph is not the right chart for unrelated categories (apples, oranges, pears — use a bar chart). Line graphs need an order along the bottom — usually time.

Reading a single point

Pick the point you want — say, the value on a Wednesday.

  1. Find Wednesday on the horizontal axis.
  2. Go straight up until you hit the line.
  3. Slide straight left to the vertical scale.
  4. Read off the number.

Watch the scale, exactly like with a bar chart. If gridlines are every 5, a point halfway between 20 and 25 is 22 or 23 — round to the closest gridline if unsure.

A worked example

Daily temperature at noon over one week:

| Day | Temp (°C) |

|---|---|

| Mon | 12 |

| Tue | 15 |

| Wed | 18 |

| Thu | 14 |

| Fri | 16 |

| Sat | 20 |

| Sun | 17 |

Drawn as a line graph with the days along the bottom and °C up the side, scale 0, 2, 4, …, 20.

Answer:

Q. On which day was it the hottest?

The highest point is on Saturday (20 °C).

Q. By how much did the temperature drop between Wednesday and Thursday?

Wed = 18, Thu = 14. Drop: 18 − 14 = 4 °C.

Q. Between which two days did the temperature rise the most?

Look at each rise:

  • Mon → Tue: +3
  • Tue → Wed: +3
  • Wed → Thu: −4 (a fall)
  • Thu → Fri: +2
  • Fri → Sat: +4
  • Sat → Sun: −3

The biggest rise is Friday → Saturday with +4.

Rising, falling, level

A line graph speaks in three shapes:

  • Up — the value is increasing.
  • Down — the value is decreasing.
  • Flat — the value is staying the same.

In Year 4 you describe a graph using these three words: "the temperature rose, then fell, then rose again."

Two lines on one graph

Sometimes a line graph shows two lines — one in red, one in blue — for two different things. Read each line with its own colour and look for places where one is higher, lower or crosses the other.

Two children's heights over a year, both starting near 130 cm:

  • Pat grows steadily from 130 cm to 138 cm.
  • Sam grows quickly from 128 cm to 142 cm.

The lines start with Pat above, cross around the middle of the year, and end with Sam above. The crossing point is when they were the same height.

Common mistakes

  • Ignoring the scale. A small step up the page may be just 1 unit (scale of 1) or 100 units (scale of 100). Always check.
  • Reading the wrong axis. Time on the bottom, quantity on the side. Going up means "more", going right means "later".
  • Joining points with a wild curve. Year-4 line graphs always use straight lines between consecutive points. The slope between two days is just a straight line — we don't know exactly what happened in between.

What's next

Try it out