Be able to add a second axis
16 Graphs and charts – Adding a Second Axis
Why a Second Axis?
Imagine you’re comparing the growth of two plants. One plant grows in centimeters, the other in inches. If you plot both on the same chart using only one axis, the scales clash and the data becomes confusing. A second axis lets each plant use its own scale, making the comparison clear.
Step‑by‑Step: Adding a Second Axis in Excel
- Enter your data – Put the first variable in column A, the second in column B.
- Insert a chart – Highlight the data and choose Insert → Scatter (X, Y) Chart.
- Add a secondary axis – Click on the data series you want on the second axis, right‑click, and select Format Data Series → Plot Series on Secondary Axis.
- Adjust the scales – Double‑click each axis to set minimum, maximum, and major unit values so the data fits nicely.
- Label clearly – Add titles, axis labels, and a legend so viewers know which line belongs to which variable.
Analogy: Two Rivers in One Map
Think of a map that shows both the Amazon (in kilometers) and the Danube (in miles). Each river needs its own scale to show its true length. The map uses two separate rulers—one for each unit—so travelers can read both accurately.
Example: Temperature vs. Time with Two Axes
| Time (hrs) | Temperature (°C) | Temperature (°F) |
|---|---|---|
| 0 | 20 | 68 |
| 1 | 22 | 72 |
| 2 | 25 | 77 |
In the chart, the left Y‑axis shows °C, while the right Y‑axis shows °F. This lets students see how the same temperature changes in two different units.
Exam Tip Box
Tip: When you’re asked to “add a second axis,” remember to:
- Identify which data set needs its own scale.
- Use the Plot Series on Secondary Axis option.
- Check that both axes have appropriate titles and units.
- Ensure the legend clearly distinguishes the two data series.
📝 Practice: Create a chart with sales (in £) and profit margin (in %). Add a second axis for the margin.
Quick Recap
- Second axes are useful when data sets use different units or scales.
- In Excel, right‑click the series → Format Data Series → Plot Series on Secondary Axis.
- Always label both axes and include a legend.
- Use colour coding to keep the chart readable.
With these skills, you’ll be ready to create clear, informative charts that compare multiple data sets—exactly what the IGCSE ICT exam expects! 🚀
Revision
Log in to practice.