Bar charts for categories, line graphs for continuous data, and what to do when a question asks for best fit.
Practice data: Categories, and data that asks for a line or curve of best fit.
Bar chart or line graph?
If the independent variable is a category (type of fruit, blood group, fuel), draw a bar chart: separate bars, drawn with a ruler, that do not touch. The y-axis starts at 0.
If the independent variable is continuous (time, temperature, concentration), draw a line graph.
Dot to dot, or best fit?
In biology the default is to join the points with straight lines using a ruler, point to point. Only draw a line of best fit when the question asks for one.
A straight line of best fit is drawn with a ruler, with equal numbers of points above and below it. It does not have to go through the origin or through any particular point.
A curve of best fit is one smooth curve, drawn freehand in a single sweep, passing through or close to the points with them balanced on either side. Never join dot to dot when a curve is asked for.
Read the instruction: "join the points" means ruled dot to dot; "line of best fit" means one ruled line with points balanced either side.
The SLAPUK mark scheme
SLAPUK
Out of 5 (S L A P U). Out of 6 (S L A P U K) when there is more than one data set on the graph.
S
Scale
Even steps of 1, 2, 5 or 10 (×10ⁿ), and the data uses more than half the grid each way. Never use the raw data as your scale.
L
Line
Join the points with straight lines using a ruler, point to point. Best fit only when the question asks.
A
Axes
What you changed goes on x, what you measured goes on y. Copy the table headings as the labels.
P
Plots
Every point within half a small square, drawn as a small × or ⊙.
U
Units
Units after a slash or in brackets: "Time / min" or "Temperature (°C)".
K
Key
Only for two or more data sets: a key that says which line is which.
Practice graphs
1. Sugar in fruit
The mass of sugar in 100 g of four fruits was measured.
Draw a bar chart of mass of sugar in 100 g for each type of fruit. Draw the bars with a ruler.[5 marks]
Results Illustrative data
Type of fruit
Mass of sugar in 100 g / g
Apple
10
Banana
12
Grape
16
Orange
9
1 · Axes, labels and scales
Put Type of fruit (the independent variable) on the x-axis and Mass of sugar in 100 g / g on the y-axis. This grid is 8 big squares across and 10 up; each big square has 5 × 5 small squares.
Set the y-axis scale, then tap above each category to draw its bar.
Keyboard and screen-reader entry
Values are placed using the scale you set, so a wrong scale still gives a wrong graph – just like on paper.
Stuck on the scale?
Fruit is a category: bar chart. y from 0: 16 ÷ 10 → 2 g per big square.
