Choose the graph from the type of each variable. If the variable you changed is a category, draw a bar chart. If both variables are measured numbers, draw a line graph or scatter graph with a best-fit line.
This is a core skill in cross-science data interpretation, because one exam paper can mix biology, chemistry and physics tables. All data here is invented for practice.
What decides the graph type?
Ask two questions. First, is the independent variable a category (names, types, groups) or a number (time, concentration, mass)? Second, if it is a number, can it take any value in a range, or only separate values?
| Independent variable | Dependent variable | Display |
|---|---|---|
| Category | Number | Bar chart, gaps between bars |
| Number, any value in a range | Number | Line graph or scatter graph with line of best fit |
| Number, grouped into classes | Frequency | Histogram or bar chart (check the question wording) |
Step by step
- Name the independent and dependent variables. Write the units.
- Check the type of the independent variable using the table.
- Choose axis scales so the plotted points fill more than half of the grid in both directions. Use even steps such as 2, 4, 6.
- Plot carefully with small crosses or dots, then draw the line or bars.
- Add a title or clear axis labels that include units.
Worked example
A student studies how the temperature of hot water changes while it cools in an open beaker. Invented data:
| Time (min) | 0 | 2 | 4 | 6 | 8 | 10 |
|---|---|---|---|---|---|---|
| Temperature (°C) | 80 | 68 | 59 | 52 | 47 | 43 |
Step 1: the independent variable is time (min), the dependent variable is temperature (°C).
Step 2: both are numbers that vary continuously, so a line graph suits. Time goes on the x-axis.
Step 3: x runs 0 to 10 min in steps of 2. The y-axis can run from 40 °C to 80 °C in steps of 10, so the points fill the grid instead of crowding in one corner.
Step 4: plot the six points and draw a smooth curve. It falls steeply at first (12 °C in the first 2 min) and flattens (4 °C in the last 2 min). A straight line of best fit would hide that shape.
The reading from the graph: the fall is faster when the water is hotter. That is an interpretation, not just a description.
A different task compares the mass of three metal samples by type. Type of metal is a category, so that needs a bar chart.
The mistake to watch for
Mistaken answer: a line graph joining “copper, iron, zinc” with a sloping line.
A line between categories implies there are values in between, which makes no sense for metals. The correction is to draw separate bars with gaps.
A second slip is starting the y-axis at 0 when all values lie between 40 and 80. That squashes the pattern into one corner of the grid.
Check yourself
1. Which graph suits “number of seeds that germinate in four soil types”?
Show answer
Soil type is a category, so a bar chart with gaps between the bars.
2. Which graph suits “current through a resistor as the voltage changes from 0 to 6 V”?
Show answer
Both variables are numbers, so plot a scatter graph with a line of best fit. Voltage goes on the x-axis.
3. Your temperatures run from 61 °C to 79 °C. Why is a y-axis from 0 to 100 a poor choice?
Show answer
The points would fill less than half the grid vertically and the pattern would look flat. Use something like 60 to 80 °C.
Where this leads next
Return to comparing datasets in different units if the data itself is the problem, or move on to explaining an anomaly without deleting it. The scientific investigation critic prompts you to justify your choice.
When a student’s graph is correct but the reasoning behind it is vague, one-to-one attention helps. Our online Combined Science tuition can work on that with your own tables.