Axes that cross zero, and two data sets on the same axes with a key (the K mark).
Practice data: Values below zero, and pairs of data sets.
Negative values
When some values are negative (a loss in mass, a temperature below 0 °C), the axis must run below zero. Use the whole range, smallest to largest, in the scale method: −17 to 18 is a range of 35.
The x-axis line itself may then sit part-way up the graph. Keep plotting from the scale, not from where the axis line is.
Two data sets: SLAPUK out of 6
With two data sets on one graph, choose ONE scale that fits both: use the smallest and largest values across both sets. Plot one set with crosses (×) and the other with circled dots (⊙), join each set separately, and add a key.
More than one line on the same axes? Add a key: that is the K in SLAPUK.
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. Potato in sucrose
Potato cylinders were left in sucrose solutions for 24 hours and their percentage change in mass calculated.
Plot a graph of percentage change in mass against concentration of sucrose solution. Join the points with straight lines using a ruler.[5 marks]
Results Illustrative data
Concentration of sucrose solution / mol/dm³
Percentage change in mass / %
0
18
0.2
8
0.4
1
0.6
-6
0.8
-12
1
-17
1 · Axes, labels and scales
Put Concentration of sucrose solution / mol/dm³ (the independent variable) on the x-axis and Percentage change in mass / % on the y-axis. This grid is 8 big squares across and 10 up; each big square has 5 × 5 small squares.
Set your scales, then tap the grid to plot each point.
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?
y: −17 to 18 is 35. 35 ÷ 10 = 3.5 → 5 per big square. Start at −20 or −25.
