Which graph?
| Graph | Use it when | Example |
|---|
| Line graph | Both variables are continuous (numbers on a scale) | Enzyme activity at different temperatures |
| Bar graph | The independent variable is in categories (discrete); bars do not touch | Number of learners with each blood group |
| Histogram | Continuous data grouped into class intervals; bars touch | Height ranges in a class |
| Pie chart | Parts of a whole (percentages adding up to 100) | Composition of household waste |
The marking checklist
A typical 6-mark graph is marked like this:
- Correct type of graph (1)
- Caption / title that names both variables: “The effect of temperature on enzyme activity” (1)
- X-axis correctly calibrated and labelled with units (1)
- Y-axis correctly calibrated and labelled with units (1)
- Plotting of points: 1 mark for 1–3 correct, 2 marks for all correct (2)
- Use an even, regular scale; it need not start at zero, but show a break if it doesn’t.
- For line graphs, join the points with straight lines (do not extrapolate beyond the data unless asked).
- Two sets of data on one graph need a key.
Calculations from data
✏️ Percentage change
1A population of 400 rhinos fell to 300. % change = (new − old) ÷ old × 100.
2(300 − 400) ÷ 400 × 100 = −25%: a 25% decrease.
✏️ Rate
1A seedling grew from 12 mm to 48 mm in 6 days.
2Rate = change ÷ time = (48 − 12) ÷ 6 = 6 mm per day.
✏️ Percentage of a total and ratio
1In a cross, 75 offspring are tall and 25 short. % short = 25 ÷ 100 × 100 = 25%.
2Ratio tall : short = 75 : 25 = 3 : 1.
Always show the formula and substitution — method marks are awarded even if the final answer is wrong. Include units.
Describing and explaining a graph
- Describe = say what happens, with data: “Enzyme activity increased from 10 to 60 units as temperature rose from 10 °C to 40 °C, then decreased to 0 at 60 °C.”
- Explain = give the biological reason: “Above 40 °C the enzyme denatures — the active site changes shape and the substrate no longer fits.”
- Read values carefully from the axes and give units. If asked for a value “from the graph”, show the reading.
- Identify the optimum, maximum, and where the trend changes.