Chartjunk

If a categorical data set has only a few distinct categories, the information in it can be very simply expressed. For example, consider the gender of each student in a class of 160. The bar chart on the right only shows that there were 100 males, 62.5% of the class.

Since the information contained in a bar chart is often simple (only 2 values above), it is tempting to embellish bar charts 'artistically' to make them more visually appealing. These additions are collectively called chartjunk. Many spreadsheets, such as Microsoft Excel, make it easy to add chartjunk to bar charts.

In general, chartjunk should be avoided — it is usually easier to read information from a standard bar chart. Rather than adding chartjunk, draw the bar chart small or replace it with a frequency table.

Three-dimensional chartjunk

A common form of chartjunk is obtained by changing each bar into a 3-dimensional object. When the resulting 3-dimensional picture is rotated, it often becomes harder to compare the heights of bars and to read off values from the axes. In particular, perspective views should be avoided.

Tanzania drinking water

The diagram below was produced by Microsoft Excel to show the distribution of the sources of household drinking water in the dry season of the 2002/3 agricultural year.

Although this display is more visually appealing than the original barchart, it is now harder to assess whether number of households with water from protected springs is over 200,000.


Although the above barchart is still acceptable, the extra rotation and perspective viewpoint of the diagram below make it an extremely poor representation of the data.


Avoid drawing bar charts in 3-dimensions.


Replacing bars with objects

A second type of chartjunk is obtained by replacing the rectangular bars in a barchart with pictures of objects. This a much more serious problem since it often visually mis-represents the proportions in the different categories. Are the frequencies proportional to the heights of the objects, their areas on the paper or their 3-dimensional volumes? At a quick glance, most readers would use something between area and volume though it is usually the heights of the bars that actually determine the size of the objects in this type of diagram.

Merit raises

As part of a study of how merit pay policies are tied to employee performance, data were collected about the merit raises (measured as a percentage of salary) for 3,990 employees in a large company. The diagram below was published to summarise the data.

The use of carrots for the bars is very misleading since doubling the height (corresponding to double the frequency) corresponds to four times the area of the carrot and eight times its volume.

In particular, the employees getting under 5% merit increase seem visually unimportant, but they comprise nearly 10% of the total employees.

Using pictures of objects instead of bars in a barchart is misleading and must be avoided.

(The merit increases above are really continuous numerical values and a histogram would have been a more appropriate display. However numerical data are occasionally grouped and treated as categorical for analysis.)