Cambridge IGCSE0417

Graphs and charts

Information Communication Technology 0417 Chapter Notes

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Graphs and charts
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In this chapter

  1. 1Choosing the Right Chart Type
  2. 2Selecting Data for Your Chart
  3. 3Creating and Labelling a Chart
  4. 4Advanced: Secondary Axis Charts
  5. 5Formatting and Enhancing Charts

1. Choosing the Right Chart Type

Charts and graphs are visual representations of data. They make it much easier to see patterns, trends, and comparisons than looking at a table of numbers. The most important first step is to choose the right type of chart for the story you want your data to tell. A bad chart choice can be misleading. The three most common types are column/bar charts, line graphs, and pie charts.

Key term

Data Series: A set of related data values from a single row or column in a worksheet that are plotted on a chart.

Common pitfall

Using a line graph for non-continuous data (like different brands of car). A line connecting these points implies a relationship that doesn't exist.

Fun fact

The first pie chart appeared in 1801 in a book by William Playfair to show the proportions of the Turkish Empire located in Asia, Europe, and Africa.

Worked example 13 marks

A teacher has recorded the favourite subjects of 30 students. The subjects are Maths, Science, English, History, and Art. Which chart type is most suitable to show the popularity of each subject, and why?

  1. 1
    1. Identify the purpose: The goal is to compare the number of students who prefer each distinct subject.
  2. 2
    1. Evaluate chart types: A line graph is for data over time, which doesn't apply here. A pie chart could work, but it's best for showing proportions of a whole, and a bar/column chart is better for direct comparison of counts.
  3. 3
    1. Select the best fit: A column chart or a bar chart is the most suitable choice.
  4. 4
    1. Justify the choice: A column/bar chart is ideal because each subject is a separate category, and the chart will clearly show which subjects are more or less popular by the height/length of the bars.

Recap

  • Use a column or bar chart to compare values across different, distinct categories.
  • Use a line graph to show a trend or changes in data over a continuous period, like time.
  • Use a pie chart to show how individual parts make up a whole, representing data as percentages.
  • Choosing the wrong chart type can make your data difficult to understand or even misleading.

Quick check

  1. Which chart type is best for showing a company's profit each month for a year?1 mark
  2. Which chart is best for showing the percentage breakdown of a student's weekly screen time between different apps?1 mark

2. Selecting Data for Your Chart

To create a chart, you must first tell the software which data to use. This data might be in one single block, or it might be in several separate areas of your spreadsheet. You need to know how to select both types. Contiguous data is in a single, unbroken block. You select it by clicking the first cell and dragging to the last cell. Non-contiguous data is in two or more separate blocks. To select it, you select the first block, then hold down the CTRL key (or Command on a Mac) and select the other required blocks.

Key term

Non-contiguous: Describes data that is located in separate, non-adjacent ranges in a worksheet.

Worked example 13 marks

A spreadsheet contains a list of products in column A, their cost price in column B, and their sale price in column D. How would you select only the product names and their sale prices to create a chart?

  1. 1
    1. Identify the data ranges: The product names are in column A and the sale prices are in column D. These are non-contiguous.
  2. 2
    1. Select the first range: Click and drag to select all the product names in column A, including the header.
  3. 3
    1. Hold the control key: Press and hold the CTRL key (or Command on a Mac).
  4. 4
    1. Select the second range: While still holding CTRL, click and drag to select all the sale prices in column D, including the header.
  5. 5
    1. Release the key and mouse: Both ranges are now selected, and you can proceed to insert a chart.

Recap

  • Contiguous data is in a single block and can be selected by clicking and dragging.
  • Non-contiguous data is in separate blocks.
  • To select non-contiguous data, select the first range, then hold CTRL (or Cmd) while selecting subsequent ranges.
  • Always include the row/column headers in your selection so they can be used as labels automatically.

Quick check

  1. What is the name for data that is all in one block, for example, cells A1 through C10?1 mark

3. Creating and Labelling a Chart

Once your data is selected, you can create a chart, usually from the 'Insert' menu. An unlabelled chart is useless. To make it understandable, you must add several key components. A Chart Title should give a clear, concise summary of what the chart shows. Axis Titles should label what is being measured on the vertical (Y-axis or Value Axis) and horizontal (X-axis or Category Axis), including units like '($)' or '(mm)'. A Legend is essential if you have more than one data series, as it explains what each colour or pattern represents.

Key term

Legend: A key that identifies what each data series (e.g., each coloured line or bar) on a chart represents.

Examiner insight

Examiners specifically award marks for meaningful chart and axis titles. Default titles like 'Chart Title' or 'Axis Title' will score zero marks.

Fun fact

Research shows people are more likely to trust data presented in a chart than the exact same data presented in a table. A good chart is a powerful tool of persuasion!

Worked example 14 marks

You have created a column chart showing the sales for 'Apples', 'Bananas', and 'Oranges'. The chart has no labels. Describe the steps to add a suitable title and axis labels.

  1. 1
    1. Add Chart Title: Locate the 'Chart Elements' option (often a '+' sign next to the chart). Tick 'Chart Title'. Click on the new title box and type a descriptive title, for example, 'Fruit Sales in May'.
  2. 2
    1. Add Axis Titles: In 'Chart Elements', tick 'Axis Titles'.
  3. 3
    1. Label Horizontal (Category) Axis: Click the horizontal axis title box and type 'Fruit Type'.
  4. 4
    1. Label Vertical (Value) Axis: Click the vertical axis title box and type 'Number of Sales' or 'Sales ($)' depending on the data.

Recap

  • Create a chart by selecting data and choosing a chart type from the 'Insert' tab.
  • Every chart must have a clear and descriptive Chart Title.
  • The vertical axis (Y-axis) shows the values and must have a title with units.
  • The horizontal axis (X-axis) shows the categories and must have a title.
  • If you plot multiple data series, you must include a Legend.

Quick check

  1. What is the name for the vertical axis on a chart, and what does it typically show?2 marks

4. Advanced: Secondary Axis Charts

Sometimes you need to plot two data series that have very different scales on one chart. For example, plotting monthly rainfall (measured in mm) and average temperature (measured in °C). If you use a single Y-axis, one data series will look flat and be unreadable. The solution is a combination chart with a secondary axis. One data series is plotted against the primary Y-axis (on the left), while the second data series is plotted against a new secondary Y-axis (on the right). Often, the second series is changed to a different chart type, like a line over columns.

Key term

Secondary Axis: A second value axis (Y-axis) on the right side of a chart, used to display a data series with a different unit or scale from the primary axis.

Examiner insight

Candidates who correctly create and label a combination chart with a secondary axis are demonstrating a high-level skill that is frequently tested and can earn many marks in a single task.

Worked example 15 marks

A dataset shows monthly 'Revenue ($)' (ranging from $50,000 to $80,000) and 'Number of Staff' (ranging from 10 to 15). You create a column chart, but the 'Number of Staff' columns are barely visible. How do you fix this?

  1. 1
    1. Right-click on the data series for 'Number of Staff' (the tiny columns).
  2. 2
    1. Select 'Format Data Series' from the context menu.
  3. 3
    1. In the options pane, choose 'Plot Series On' and select 'Secondary Axis'. The columns for staff numbers will now be taller, plotted against a new axis on the right.
  4. 4
    1. To improve clarity, right-click the 'Number of Staff' series again, select 'Change Series Chart Type'.
  5. 5
    1. In the dialog box, change the chart type for 'Number of Staff' to 'Line' and ensure the 'Secondary Axis' box for it is ticked. This creates a combination chart.
  6. 6
    1. Finally, add an axis title for the new secondary axis on the right, labelling it 'Number of Staff'.

Recap

  • Use a secondary axis when plotting two data series with very different scales or units.
  • To create one, format a data series to be plotted on the 'Secondary Axis'.
  • It is common to change the chart type of the secondary series to a line to create a 'combination chart'.
  • You must add a descriptive title for the secondary axis, just as you do for the primary axis.
  • Failure to add a secondary axis can make one of your data series unreadable.

Quick check

  1. On which side of the chart does the secondary axis typically appear?1 mark

5. Formatting and Enhancing Charts

A standard chart gets the job done, but formatting can make it clearer, more professional, and more impactful. You can modify almost any element. Common enhancements include changing the axis scale (e.g., making the Y-axis start at 50 instead of 0 to zoom in on variations), formatting numbers on the axis (e.g., to show currency symbols or a specific number of decimal places), adding data labels to show the exact value of each bar or point, and changing colours or patterns to match a corporate style or highlight a key piece of data. For pie charts, you can 'explode' a single slice to draw attention to it.

Key term

Data Labels: Text added to a data point or bar on a chart that displays the exact value from the worksheet.

Common pitfall

Changing the axis scale in a way that misleads the viewer. For example, starting a Y-axis at a high value can exaggerate small differences, which can be deceptive.

Worked example 14 marks

You have a bar chart showing student test scores, which range from 65% to 90%. The Y-axis currently starts at 0. How and why would you change the axis scale? Also, how would you add the exact scores to the bars?

  1. 1
    1. To change the scale, right-click on the vertical (Y) axis and select 'Format Axis'.
  2. 2
    1. In the Axis Options, find the 'Bounds' section. Change the 'Minimum' value from 0 to a value closer to the lowest data point, for example, 60.
  3. 3
    1. The reason for this change is to make the differences between the scores more apparent and easier to see. A scale from 0-100 would make all the bars look similarly tall.
  4. 4
    1. To add exact scores, click the chart, go to 'Chart Elements' (the '+' icon), and tick the box for 'Data Labels'. The exact value will now appear on or near each bar.

Recap

  • Format chart axes by right-clicking on them and selecting 'Format Axis'.
  • You can change the minimum, maximum, and major interval values of an axis scale.
  • Add Data Labels to show the exact value of each point on your chart.
  • Enhance appearance by changing the fill colour/pattern of data series or the chart background.
  • In a pie chart, a slice can be 'exploded' (pulled out) to emphasize it.

Quick check

  1. What is the term for pulling a single slice of a pie chart away from the center?1 mark
  2. Where would you go to change the Y-axis numbers to show a '$' symbol?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Explain the difference between a bar chart and a line graph, and give a suitable example of data for each.4 marks
  2. What is the purpose of a chart legend and a chart title?2 marks
  3. Describe the steps you would take to select data in columns A and F of a spreadsheet to create a chart.3 marks
  4. A scientist has recorded the temperature of a liquid every minute for one hour. The values range from 85°C to 95°C. When she creates a line graph, the line looks almost flat. Explain how she could format the chart to show the temperature variation more clearly.3 marks
  5. Explain what a secondary axis is and why you would need to use one.3 marks
  6. You are given a pie chart showing a company's sales by region. Describe two ways you could enhance the chart to make the largest region's sales stand out.2 marks
  7. A spreadsheet file 'RATES.csv' contains data for 'Average Heart Rate' and 'Average Breathing Rate' for a group of athletes during exercise. Create a single chart that effectively displays both sets of data. The heart rate is between 120-180 bpm and the breathing rate is between 30-50 breaths per minute. Justify your choice of chart and features.6 marks
  8. What is the difference between contiguous and non-contiguous data?2 marks
  9. Describe how you would format the value axis of a chart to display numbers as currency with two decimal places, for example, $1,234.50.3 marks
  10. A column chart shows the population of five cities. Explain how to add labels to the top of each column showing the exact population figure.2 marks

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