Graphical Presentation Of Tabular Data - Bar And Cylinder Charts (part 3 Of 5)

This is the third article in a series aimed at showing the benefits of presenting tabular data in graphical format

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This article considers the use of vertical column, horizontal bar and cylinder charts and shows how they should be used, how they are constructed, and the benefits that they can provide both to the presenter and their audience. This form of chart is for plotting and displaying discrete data such as the hat sizes of people or the colour of their eyes.

A survey was cited in parts one and two of this series of articles and it is again used and extended for the purpose of this article. The continuous data of 25 children's heights obtained for producing the histogram in article 2 can now be grouped in order to produce discrete or discontinuous data for which vertical, horizontal and cylinder charts are primarily designed.

Vertical Column Bar Charts

This visual representation of data is a chart where data values are represented by vertical columns whose heights are equivalent to each of the values of one of the survey's criteria and all these vertical heights are proportional to each other. The width of the column is immaterial and is usually the same for each value displayed.

From the original survey a frequency table needs to be produced from the sample of 25 children used. A decision needs to be made of how to group the height data and what increments should be used. For the purpose of this article an increment of 0.1m has been chosen so that the grouping of the heights in metres shall be 1.0 to 1.1, 1.1 to 1.2, 1.2 to 1.3 etc up to 1.7 to 1.8. Each child's height is considered in turn and it is determined into which height range their height falls. When all the children's heights have been allocated into a height range, then it is known how many children fall within each height range. This is known as the frequency that each height range occurs. The survey may have produced a table, such as the one shown below:

Height Range (m.) / Frequency (No.)

1.1-1.1 /0

1.1-1.2 /1

1.2-1.3 /4

1.3-1.4 /9

1.4-1.5 /7

1.5-1.6 /3

1.6-1.7 /1

1.7-1.8 / 0

Total 25

Using this reorganised data, a vertical bar chart can be created. The vertical axis of the chart, also known as the 'y axis', would represent the frequency that each height range occurred within the survey and would range from 0 to 10. This ensured that all the frequency values of 0 to 9 were contained within the chart. The horizontal axis, or 'x axis', would represent the height range increments used in the survey and would therefore range from 1.0m to 1.8m in 0.1m steps.

For each increment of height a vertical column is drawn which is equivalent in height to the survey frequency result. For example, for the 1.2m to 1.3m height range on the horizontal axis, a column is drawn to a height of 4 on the vertical axis and a value of 7 for the 1.4m to 1.5m height range value. The height values for the 1.0m to 1.1m and 1.7m to 1.8m ranges would be zero as no child's height fell within these ranges. When the vertical bar chart has been completed for all the height range values, a series of vertical columns can be viewed, each representing the frequency that each height range value occurs. By drawing horizontal lines across the chart, the actual frequency that each height range occurs can be easily read off the vertical axis of the chart. The individual columns can be displayed in different colours to enhance the visual display.

The representation of data in the form of bar charts is more pleasing to the eye and more understandable than the original spreadsheet table. The person viewing the chart can determine visually far more from the graph, not just that the second most frequent height range was 1.4m to 1.5m but more children were higher than the most frequent height range of 1.3m to 1.4m. Whilst these conclusions can also be achieved from the original table, the person interpreting the chart obtains a better understanding of the dispersal of the survey data.

Each vertical column within the bar chart could be enhanced by subdividing each frequency into a girl and boy frequency. The two resulting frequency elements for each height range could be displayed using two different colours, say red for the boys and yellow for the girls. If the boys' frequency is always displayed at the bottom of each column, the information displayed adds a new dimension and can be better understood and interpreted by the survey audience.

Horizontal Bar Charts

Horizontal bar charts are used in the same way as vertical bar charts except here the data is represented by horizontal bars instead of vertical columns.

Using our survey frequency sample again to demonstrate this technique, the horizontal and vertical axes of the horizontal bar chart are reversed, so that the frequency is now on the horizontal axis and the height ranges are represented by the vertical axis. The chart is produced as before but instead of vertical columns, horizontal bars are produced.

It is mostly a matter of personal choice as to when vertical columns or horizontal bars are used. However, there are instances when the use of the horizontal bar chart is preferable. Such a case is in project planning where Gantt Charts are used to enable critical path analysis to be carried out. Here the horizontal axis always represents time, and each task element of the project is represented on the vertical axis. Engineering and building projects can take many months and/or years to complete and can contain many hundreds of individual tasks. A horizontal bar chart enables an engineer to plan and construct a project in the correct sequence of tasks, ensure tasks are completed before others commence and complete a project in order, and within time schedules.

Cylinder Charts

These charts are very similar to vertical column charts and are constructed in the same way. There are however two main differences. The first is that instead of representing a value by a two dimensional vertical rectangular column, a representative three dimensional cylinder is used. In addition whereas in the vertical bar chart adjacent bars touch each other, the cylinders within cylinder charts mostly have a space between them. Cylinder charts are generally shown having a base on which the cylinders sit on and the vertical axis is portrayed as a wall. Zero values also show up more clearly as ovals on the base axis.

When considering which type of chart is best suited to represent the data you wish to display, it is important to ensure that the chart chosen not only aids you in your presentation, but also aids the audience in understanding the message you are trying to convey to them. It is no good showing off your skills in chart production and presentation if you don't get your message across to your audience or perhaps even confuse them.

The next article in this series considers the use of scatter and line graphs and when they should be used, how they are constructed and the benefits that they can provide.

by: Paul E West

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