Graphs: Choosing, Drawing & Reading Them

Revise Graphs: Choosing, Drawing & Reading Them for Biology 3 (IAL) WBI13 (AS Level) — revision notes, 2 practice questions and instant AI marking.

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Edexcel IAL Biology • Unit 3: Practical Skills in Biology I

Graphs: Choosing, Drawing & Reading Them

📈 Big idea: Choosing the wrong type of graph loses marks before you draw a single point. Continuous independent variable means a line graph; categories mean a bar chart. After that, most of the marks are mechanical — axes, labels, units, scale, plotting, line — and every one of them is yours for the taking.

Summary — What This Topic Covers

Choosing between a line graph, a bar chart and a scatter graph
Which variable goes on which axis, and why
Choosing a scale that uses the paper
Drawing a line of best fit — and when not to join the points
Reading a gradient, and taking values from a curve

1. Choosing the Right Graph

IV IS… USE EXAMPLE ───────────────────────────────────────────────────────────────────────── continuous (any value in a range) line graph temperature, pH, concentration, time categoric (named groups) bar chart species, tissue type, light/dark two continuous, neither controlled scatter graph height vs mass (+ line of best fit) in a population frequency of continuous data histogram number of leaves in (bars touch) each length band

The distinction

A bar chart has gaps between the bars because the categories are separate. A histogram has no gaps because the data is continuous and the bars represent adjoining ranges.

Common mistake

Drawing a bar chart for temperature. Temperature is continuous — 25 °C exists between 20 and 30, so the bars would be hiding real values. Continuous data always gets a line.

2. Axes

The rule

Independent variable on the x-axis (horizontal), dependent variable on the y-axis (vertical). What you changed goes across; what you measured goes up.

Label both axes with quantity / unit, exactly as in the table
Choose a scale that makes the plotted points fill at least half of each axis
Use sensible intervals — 1, 2, 5, 10 and their multiples. Never 3, 7 or 15
The axes do not have to start at zero, but say so if they do not

✗ points crammed into the bottom-left corner → scale mark lost ✗ x-axis in steps of 3 → hard to read between points ✓ points spread across most of the grid, steps of 10

3. Plotting and the Line

Plot each point as a small neat cross or an encircled dot — a blob hides where the value actually is
Draw a line of best fit where the relationship is expected to be smooth: a single smooth line with roughly equal numbers of points either side
Do not join the points dot-to-dot when a trend is expected — that implies the wiggles between points are real
Do join point to point only when each reading is a separate measurement with no expected trend between them

Anomalies on a graph

Plot the anomalous point, circle it, and ignore it when drawing the line of best fit. Do not leave it out of the graph — that is hiding data.

What mark schemes look for

4. Reading a Gradient

The method

on the line

5. Describing What a Graph Shows

Practice Questions

Practice Question 1

Practice Question 2

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