Be able to add a second axis

Topic 16 – Graphs and Charts

Objective: Be able to add a second axis

In many real‑world data sets two different quantities need to be compared on the same chart. Adding a second (vertical) axis allows each data series to be plotted using its own scale, making the chart clearer and more informative.

1. What is a second axis?

A second axis is an additional vertical (y) axis placed on the opposite side of the chart. It is used when the two data series have different units or ranges.

2. When should a second axis be used?

• When the two data series have different units (e.g., temperature °C vs. rainfall mm).
• When the ranges differ significantly, causing one series to appear flat on a single‑axis chart.
• When you need to show the relationship between two variables that are not directly comparable on the same scale.

3. Steps to add a second axis in a spreadsheet

1. Select the chart that already displays the first data series.
2. Insert the second data series into the chart (usually via “Select Data” or “Add Series”).
3. Right‑click the newly added series and choose “Format Data Series”.
4. Set the series to plot on the “Secondary Axis”.
5. Adjust the scale of the secondary axis (minimum, maximum, major unit) so the data is displayed clearly.
6. Label both axes with units and titles to avoid confusion.
7. Optionally, change the series colour or line style to differentiate the two data sets.

4. Example

Suppose a school records the average temperature (°C) and the number of students absent each month. The temperature ranges from 5 °C to 25 °C, while absences range from 0 to 120. A single axis would make the absence line appear almost flat. Adding a second axis solves this.

Data Table

Interpretation

The chart shows a clear inverse relationship: as temperature rises, the number of absences falls. This can be expressed mathematically as a negative correlation, approximated by the linear regression equation:

\$\text{Absent} = -4.5 \times \text{Temp} + 70\$

where “Temp” is the average temperature in °C.

5. Common pitfalls

• Forgetting to label the secondary axis – students may misinterpret the data.
• Using the same colour for both series – makes it hard to distinguish them.
• Setting inappropriate axis limits – can exaggerate or hide trends.

6. Practice Questions

1. Given the data below, create a chart with a secondary axis. Identify which series should be on the primary axis and which on the secondary axis.
   

   Quarter	Revenue (£000)	Units Sold
   Q1	120	800
   Q2	150	950
   Q3	180	1100
   Q4	210	1300

2. Explain why a secondary axis is preferable to a stacked bar chart for the temperature/absence example.
3. In a chart showing “Hours Studied” (0–20 h) and “Test Score” (0–100 %), which axis should be primary? Justify your choice.

7. Summary

Adding a second axis is a powerful technique for comparing two data series with different units or scales. By following the steps outlined, you can create clear, accurate charts that convey relationships effectively.