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Plotting graphs: axes, scales, points and best-fit lines

Edexcel IAL Physics · WPH13 Practical Skills in Physics I

A graph question is often worth 5 or more marks. Examiners use a fixed checklist, so you can earn every mark by following it.

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Axes and labels

Plot the quantity asked for on the y-axis against the one on the x-axis: 'plot T² against l' means T² up, l along.
Label each axis with quantity and unit separated by a slash: T² / s², I / A, 1/d² / m^-2.
Axes do not need to start at zero.

Scales

Each 2 cm square should represent 1, 2 or 5 units (times a power of ten): 0.1, 0.2, 0.5, 1, 2, 5, 10...
Avoid 3, 4, 7, 8 or 0.25 per square: they cause plotting errors and lose the mark.
Choose the scale so the points cover more than half of the grid in both directions.
Label every major (2 cm) gridline.

Points

Plot each point as a small, neat cross (x or +), accurate to within 1 mm (half a small square).
Large dots are not credited, as their centre is unclear.
Recheck any point that sits far from the trend: it is often a plotting slip.

Line of best fit

Use a single 30 cm ruler for a straight line: thin, continuous, no kinks.
Balance the points either side of the line along its whole length.
Do not just join the first and last points, and do not force it through the origin.
If you ignore an anomalous point, circle it and say so; every point not marked as anomalous must be considered.

Worked example

x-values run from 0 to 0.85 m. The grid has 8 large squares (2 cm each) across. Choose a scale.

0.85 / 8 = 0.106 m per square, so 0.1 m per square is too small (only reaches 0.8 m).
Next standard value: 0.2 m per square reaches 1.6 m.
With 0.2 m per square the points use 0.85 / 1.6 = 53% of the width: more than half, so acceptable.

Answer: 0.2 m per 2 cm square

Key idea: y against x as asked; quantity / unit labels; scales of 1, 2 or 5 per square filling over half the grid; small crosses within 1 mm; one thin ruled line balancing the points; circle anomalies.

Check yourself

Which scale for 2 cm squares would examiners accept?
- 3 N per square
- 0.5 N per square
- 0.25 N per square
- 7 N per square
Show answer
0.5 N per square. 0.5 is 5 x 10^-1; 3, 0.25 and 7 make plotting awkward.
Which axis label is in the expected form?
- Time squared
- T² (s)
- s²
- T² / s²
Show answer
T² / s². Quantity and unit, separated by a slash.
Data for the y-axis run from 0 to 3.4 V. The grid has 7 large squares up. Choose the smallest standard scale (from 0.1, 0.2, 0.5, 1, 2 V per square) that fits all the data. Enter the scale in V per square.
Show answer
0.5 V per square. 3.4 / 7 = 0.486 V per square; 0.2 is too small (reaches 1.4 V); 0.5 reaches 3.5 V.
Which is the correct way to draw a straight line of best fit?
- A line joining the first and last points
- A line forced through the origin
- A single thin ruled line with points balanced either side
- Short straight segments joining each point
Show answer
A single thin ruled line with points balanced either side. The line must represent the trend of all the data.
Why should the scale make the points cover more than half the grid in each direction?
- It lets points be plotted and read more precisely
- Examiners prefer large numbers
- It forces the line through the origin
- It removes anomalous points
Show answer
It lets points be plotted and read more precisely. A compressed graph makes each small square represent too much, so plotting and gradient readings are imprecise.
Must the axes of a graph always start at zero?
- Yes, always
- Only the x-axis must start at zero
- No; start them where the data needs them, unless an intercept must be read
- Only for curves
Show answer
No; start them where the data needs them, unless an intercept must be read. A false origin is fine and often makes better use of the grid; just take care when finding intercepts.
A plotted point lies far from the line. What should you do first?
- Check that it has been plotted correctly
- Draw the line through it
- Delete it from the table
- Start the graph again
Show answer
Check that it has been plotted correctly. Examiners note that many apparent anomalies are plotting slips. If it is plotted correctly and still anomalous, circle it.

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