Visualisation: Visual representations of data and information

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# 2 The most common spreadsheet charts

In this section, I’m assuming that you are familiar with three types of charts provided by spreadsheets – bar charts, pie charts, and line charts (often referred to as ‘line graphs’ or just ‘graphs’) – and know how to use a spreadsheet to produce them.

## Types of charts

Each chart type communicates information differently to the chart reader. (Or should that be ‘chart viewer’? The terms will be used interchangeably.)

• The pie chart, as shown in Figure 1(a) below, can be used to represent proportions of a whole. For example, if you have set of non-overlapping, percentage-based results that add up to 100%, and not too many categories, it might be appropriate to use a pie chart to represent the results in a visual way.
• The bar chart, as shown in Figure 1(b) below, can be used to compare data obtained from independent members of a set, such as the population size for each country in the set of countries in the European Union.
• The line chart, as shown in Figure 1(c) below, is often used to plot the behaviour of a numerical quantity over time (in which case the data may be described as ‘time-series data’). More generally, line charts can be used to plot two continuous variables against each other.
Figure 1 (a) a simple pie chart; (b) a simple bar chart; (c) a simple line chart

## Activity 3 (self-assessment)

Which chart type would you choose for each of the following data sets?

• a.The number of mobile phone minutes called in the UK recorded on a monthly basis over the last year.
• b.The number of mobile phone subscribers in the UK recorded on a monthly basis over the last year.
• c.The number of mobile phone subscribers in the UK recorded on a monthly basis over the last five years, with the purpose of revealing the trend and making a forecast for the next three years.
• d.The relative market share in terms of subscribers of the different mobile phone operators.

### Comment

Here are my answers, with reasons.

• a.A bar chart would be an appropriate way of displaying this information, on the assumption that usage in one month is independent of usage in another month.
• b.Although you could use a bar chart here, a line chart would be more appropriate, because a significant number of subscribers in one month are likely to be continuing subscribers from a previous month and so the number of subscribers in one month is not really independent of the number in the previous month.
• c.Line charts are the best adapted to revealing trends and making forecasts, so a line chart would be most suitable here.
• d.As we are talking about proportional market share – that is, the share of the whole market held by each company – it would make sense to use a pie chart to display this information, using separate segments for not more than the four or five dominant providers and a single ‘other’ segment to represent the rest.
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