Be able to add a second data series

Topic 16 – Graphs and Charts

Objective: Be able to add a second data series

In many real‑world situations you need to compare two sets of related data on the same graph. Adding a second data series allows you to see trends, relationships and differences more clearly.

1. When to use a second data series

• Comparing performance of two products over time.
• Showing actual vs. projected values.
• Displaying two related variables, such as temperature and rainfall.

2. Types of charts that support multiple series

• Line chart – each series is a separate line.
• Column/Bar chart – each series appears as a separate column or bar for each category.
• Area chart – overlapping shaded areas for each series.
• Scatter plot – each series is a distinct set of points.

3. Steps to add a second data series (using a typical spreadsheet program)

1. Enter the first series of data in a column (e.g., Series 1).
2. Enter the second series in the adjacent column (e.g., Series 2).
3. Select the complete data range, including both series and the category labels.
4. Insert the desired chart type (Line, Column, etc.). The program will automatically plot both series.
5. If the chart was created with only the first series:

   1. Click the chart to activate the Chart Tools menu.
   2. Choose “Select Data” (or similar).
   3. Click “Add” under “Legend Entries (Series)”.
   4. Specify the name, X‑values (categories) and Y‑values (data) for the second series.
   5. Confirm and the second series will appear on the chart.

6. Format each series as required (different colours, line styles, markers).
7. Add a clear legend, axis titles and a chart title.

4. Example data set

The table below shows monthly sales (in units) for two products, A and B, over six months.

To display this information on a line chart with two series, follow the steps in section 3. The resulting chart will have two lines, one for Product A and one for Product B, sharing the same month axis.

5. Key points to remember

• Both series must have the same number of data points (\$n\$) and correspond to the same categories.
• Use contrasting colours or line styles so the series are easily distinguishable.
• Always include a legend that clearly labels each series.
• Check that the axis scales are appropriate for both series; adjust the secondary axis only when the ranges differ greatly.

6. Common mistakes and how to avoid them

• Missing category labels: Ensure the X‑axis labels (e.g., months) are selected for both series.
• Unequal data lengths: If one series has fewer points, fill missing cells with “0” or “N/A” before creating the chart.
• Incorrect legend entries: Edit the legend manually if the default names are not descriptive.
• Overcrowded chart: Limit the number of series to a maximum of three for clarity in IGCSE examinations.

7. Practice exercise

Using the data below, create a column chart that displays the number of visitors to two museums (Museum X and Museum Y) over four quarters.

Follow the steps in section 3 to add both data series, then answer the following questions:

1. Which museum had the greatest increase in visitors from Q1 to Q4?
2. What is the total number of visitors for each museum over the year?
3. Describe one visual difference you notice between the two series on the chart.

8. Summary

Adding a second data series is a fundamental skill for creating informative charts. By correctly arranging data, selecting the appropriate chart type, and using the “Add Series” function, you can compare two related data sets effectively. Remember to label axes, provide a clear legend, and choose contrasting visual styles to make your chart easy to read.