A line graph shows how a quantity changes over time. Individual values are plotted as points on a grid and then joined with straight lines. The horizontal axis usually shows time (months, years, hours), and the vertical axis shows the measured value, so you can read off trends at a glance.
Line graph = data points plotted and joined with straight lines — shows change over time
Example 1 — Reading a value from a line graph
Step 1
Identify the axes. The x-axis shows months; the y-axis shows temperature in °C.
Step 2
To read July: find Jul on the x-axis, trace straight up to the plotted point, then read across to the y-axis → 22°C.
Step 3
To find the range: maximum (Jul) = 22°C, minimum (Jan/Dec) = 4°C. Range = 22 − 4 = 18°C.
Example 2 — Describing the trend
Step 1
Look at the overall direction of the line from left to right. Rising = upward trend. Falling = downward trend.
Step 2
The temperature rises from January to July (peak at 22°C), then falls from July to December.
Step 3
Write a sentence: "Temperature increases from January, peaks in July at 22°C, then decreases back to 4°C in December — showing a seasonal pattern."
Key exam words: "upward/downward trend", "overall increase/decrease", "peaked in [month]", "remained stable between [dates]", "fell sharply between [months]".
✓ Quick Check 1 — Trends and reading line graphs
Question 1 of 10
Drawing Line Graphs — Scale, Plotting, and Labels
Drawing an accurate line graph requires choosing a sensible, even scale for both axes. Every interval must represent the same value — you cannot change the scale partway up. Label both axes with titles and units, add a title, and join each consecutive pair of points with a straight line.
Steps: choose scale → label axes with units → plot each point → join with straight lines using a ruler → add title
Choose a y-axis scale: values range from 40 to 80 — use 0 to 90 in steps of 10. (Must include 0 and go above the maximum.)
Step 2
Label the x-axis "Day of the week" (Mon, Tue, Wed, Thu, Fri) and the y-axis "Number of customers". Add a title.
Step 3
Plot each point with a cross (×): Mon at 40, Tue at 55, Wed at 70, Thu at 65, Fri at 80.
Step 4
Join consecutive points with a ruler. Trend: overall increase with a slight dip on Thursday.
⚠ Common mistake: never change the y-axis scale halfway up the axis — every grid square must represent the same value. Also, always start at 0 unless told otherwise.
✓ Quick Check 2 — Drawing line graphs accurately
Question 1 of 10
Comparing Two Line Graphs on the Same Axes
Two data sets can be plotted on the same pair of axes using different coloured or styled lines. A key (legend) must label each line. This allows you to compare trends directly, identify when lines cross (crossover points), and state which is greater at any given time.
Example 4 — Reading a dual-line graph
Step 1
Use the key. The solid blue line is Shop A; the dashed yellow line is Shop B.
Step 2
Read January values: Shop A = 20, Shop B = 12. Shop A started with higher sales.
Step 3
Identify the crossover: the lines meet between February and March — this is where Shop B overtook Shop A briefly in March (both at approximately 15–18).
Step 4
In June: Shop A = 25, Shop B = 21. Shop A is £4,000 higher by June. Shop A recovered and pulled ahead.
Example 5 — Comparing trends
Step 1
Shop A trend: high start (20), dips to a low of 15 in March, then rises strongly to 25 in June. More variable.
Step 2
Shop B trend: starts lower (12) but rises steadily Jan–Apr, then levels off slightly. More consistent growth.
Step 3
Both shops end June higher than January, but Shop A shows greater fluctuation while Shop B shows steadier growth.
Always include a key when drawing a dual-line graph. Use different colours or line styles (solid vs dashed) and label each one in the key.
✓ Quick Check 3 — Dual-line graphs and comparison
Question 1 of 10
Practice: Reading Values from a Line Graph
A shop's monthly sales (£000s): Jan 12, Feb 18, Mar 24, Apr 20, May 28, Jun 32. Answer the questions below.
#
Question
Answer
Result
1
What were the sales in March? (£000s)
2
In which month were sales highest?
3
By how much did sales rise from January to June? (£000s)
4
Which month showed a dip (lower than the month before)?
5
What were the sales in January? (£000s)
6
What were the sales in May? (£000s)
7
By how much did sales rise from February to March? (£000s)
8
What were the total sales for January to March combined? (£000s)
9
By how much did sales rise from April to May? (£000s)
10
What were the total sales for April to June combined? (£000s)
⚠ Things to Watch Out For
Joining points with curves instead of straight lines: Connect consecutive data points with STRAIGHT LINES. Curved lines suggest continuous trend data, not discrete data points.
Wrong scale on y-axis: The y-axis scale must be consistent — equal spacing for equal value differences. An uneven scale distorts the visual impression of trends.
Not plotting points first: Always plot individual points (×) accurately before connecting them. Drawing a line without marking points leads to inaccurate graphs.
Interpolation beyond the data: Only interpolate (read values from the graph) BETWEEN plotted data points. Extending the line beyond the data range (extrapolation) gives uncertain estimates.
Misreading between gridlines: When reading a value between gridlines, estimate carefully. Each small square represents a specific value — count squares, don't guess.