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Choosing and plotting graphs

Edexcel IAL Biology · WBI13 Practical Skills in Biology I

Graph questions are high-scoring if you follow the rules, and graph plotting was one of the best-answered items in January 2025. The common slips are scales, labels and lines.

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Which graph?

Line graph or scatter graph: both variables continuous (temperature and rate). Use for a relationship or correlation.
Bar chart: IV is categoric (type of fibre, species of plant, antibiotic A or B), usually showing means, with error bars or range bars for variability.
Histogram: frequency of a continuous variable in classes (lengths of leaves), bars touching.

Bar graphs 'should have a y axis starting at zero' (January 2024 Unit 6 report).

Plotting rules

1. IV on the x axis, DV on the y axis.

2. Label both axes with quantity and unit: 'Mean rate / s⁻¹'.

3. Choose a linear scale using easy steps (1, 2, 5 or 10 per large square) so the points fill more than half the grid; do not use the data values themselves as the scale.

4. Plot points accurately with small crosses (x or +).

5. Draw a line of best fit (straight or a smooth curve) with points balanced either side, or join points with straight ruled lines if you cannot assume the trend between them. Do not draw a line through every point by hand, and do not extrapolate to zero unless you have data or reason to.

The June 2024 report: 'A line of best fit should follow the direction of the slope', with points balanced above and below.

Describing a graph

Describe the overall trend (positive, negative, levels off), quote numbers from the graph with units, and note where the pattern changes. Explain using biology only if asked to explain. A correlation alone does not prove one variable causes the other.

Worked example

Students measured the tensile strength of five different types of plant fibre. Which graph should they draw and why?

Fibre type is categoric (discrete names).
So use a bar chart of mean mass to break, y axis from zero, with range or standard deviation bars.

Answer: A bar chart, because the independent variable is categoric; plot means with error bars and start the y axis at zero.

Key idea: Continuous IV: line or scatter graph; categoric IV: bar chart (y from zero); histogram for frequencies. IV on x, labels with units, linear easy scales filling the grid, accurate crosses, balanced best-fit line, no extrapolation.

Check yourself

Which graph is suitable for mean zone of inhibition for four different plant extracts?
- Line graph
- Scatter graph
- Pie chart
- Bar chart
Show answer
Bar chart. Plant extract type is categoric.
In CP4, which variable goes on the x axis?
- Initial rate, the dependent variable
- Temperature, the independent variable
- Time taken to set up
- Volume of enzyme
Show answer
Temperature, the independent variable. IV on the x axis.
Which is the best scale choice for a y axis on graph paper with 10 large squares when data go up to 46?
- 4.6 units per large square
- 10 units per large square starting at 40
- 3 units per large square
- 5 units per large square
Show answer
5 units per large square. 5 per square uses easy steps and fits 0 to 50 on the grid.
Why should the y axis of a bar chart start at zero?
- Otherwise the heights of the bars exaggerate differences
- Bar charts cannot show negative numbers
- It makes the axis label shorter
- The x axis must also start at zero
Show answer
Otherwise the heights of the bars exaggerate differences. Bar length represents the value, so the axis starts at zero.
Which describes a good line of best fit?
- A line joining each point to the next freehand
- A line passing through the origin in every case
- A single straight line or smooth curve with points balanced either side
- A line through the first and last points only
Show answer
A single straight line or smooth curve with points balanced either side. Balanced either side, following the trend.
A student extends a line of best fit to the origin where no data were collected. Why might this lose a mark?
- There is no evidence the trend continues to zero
- Lines must always stop at the y axis
- The origin is never on a graph
- Lines of best fit must be curved
Show answer
There is no evidence the trend continues to zero. The January 2025 report noted extrapolating to zero as a common error.
Which y axis label would gain credit on a graph of means from a t-test investigation?
- Mass
- Mean
- Mean mass of leaves eaten / g
- g
Show answer
Mean mass of leaves eaten / g. The January 2024 Unit 6 report noted labels missing 'mean' and units.

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Last updated 8 November 2026. Spotted a mistake? Tell us.