1 Graphs and charts

At the end of last week you looked at how useful tables are for summarising a lot of data concisely, clearly and accurately. However, sometimes you want to get an overall message across quickly and this may well stay hidden in a table of data. In this situation you can use a graph or a chart instead. Graphs can also be used to explore relationships between sets of data, such as the way in which a sunflower grows over a season.

You have probably seen different types of graphs and charts in your day-to-day life, such as when watching the TV, reading a newspaper or on the internet. Here is a flavour of some you may have encountered.

These allow a visual comparison between different categories. For instance, in Figure 1 you can see very quickly that the hiking/hill walking category was the most popular activity, and cruising the least popular.

Figure 1 Example bar chart

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This is an example of a horizontal bar chart. The title reads ‘What activities did they engage in? (000s).’ The horizontal axis has a scale marked 0, 50, 100, 150, 200 at equal intervals. The vertical axis shows the different activities: equestrian, golf, cycling, angling, hiking/hill walking and cruising. A horizontal bar is drawn opposite each activity to represent the number of people engaging in each activity. The bars are separated from each other by a small gap. The length of each bar can be measured from the scale below although the length is also displayed at the end of each bar as follows: equestrian 29; golf 134; cycling 100; angling 83; hiking/hill walking 168; cruising 22. A dotted arrow stretches from the right-hand end of the cycling bar vertically down to the horizontal axis, intersecting the axis at 100.

Figure 1 Example bar chart

Again, these allow a quick visual comparison between categories but this time using percentages rather than the actual data. This pie chart shows very clearly that walking is the favourite activity.

Figure 2 Example of a pie chart

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This is an example of a pie chart. The title reads ‘Favorite activities’. A pie chart is a circle divided into sectors so that the area of each sector is proportional to the number of items in that sector. In this example there are five sectors each comprising of a different shade of blue. The sectors are labelled ‘Walking 40%’, ‘Swimming 18%’, ‘Cycling 24%’, ‘Golf 12%’ and ‘Other 6%’.

Figure 2 Example of a pie chart

Finally, a line graph, where all the plotted points are joined, can show clear relationships between the plotted data. In this case, the curve shows that the weight (or as it’s properly known, mass!) of a woman increases over the last six weeks of her pregnancy, but that this weight gain slows towards the end. This is shown by the flattening out of the curve in Figure 3.

Figure 3 Example line graph

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This is an example of a line graph. It is entitled ‘Weight during last six weeks of pregnancy’ and states ‘Source: Hospital records’. The horizontal axis, labelled as ‘Week of pregnancy’, is numbered from week 30 to 40. The vertical axis, labelled as ‘ Weight in lb’, is numbered from 160 to 174, starting from the bottom, where the two axes intersect. There is a smooth curved line starting at week 33 and about 163 lb that rises up towards week 40 and about 173 lb.

Figure 3 Example line graph

You may well have come across other charts or graphs, but as these are three of the most common types, bar and pie charts and line graphs will be the focus of this week. To get you started, in the next section you will look at plotting points on a line graph.