Teaching:TUW - UE InfoVis WS 2008/09 - Gruppe 07 - Aufgabe 1 - Pie Chart

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Zurueck zu Aufgabe 1

Pie Charts[edit]

Definition[edit]

A pie chart (or a circle graph) is a circular chart divided into sectors, illustrating relative magnitudes or frequencies. In a pie chart, the arc length of each sector (and consequently its central angle and area), is proportional to the quantity it represents. Together, the sectors create a full disk. It is named for its resemblance to a pie which has been sliced.
[Wikipedia, 2008]


How to use it[edit]

Pie charts are used to show percentages. The circle provides a visual concept of the whole (100%). Despite its popularity, pie charts should be used sparingly for two reasons [Statistics Canada, 2008]:

  • Use less than six components - otherwise, the resulting picture will be too complex to understand.
  • Not useful when the values of each component are similar because it is difficult to see the differences between slice sizes.


To present certain values in a pie chart it is necessary to determine how many degrees represent this share of data in comparison to the whole circle. This calculation is done by developing the equation: 

  percent ÷ 100 x 360 degrees = the number of degrees

This ratio works because the total percent of the pie chart represents 100% and there are 360 degrees in a circle [Statistics Canada, 2008].

To construct a pie chart the following steps are necessary [Concordia University, 1999]:

  • Determine the proportions: find the total value for the entire category being studied and calculate the percentage for each segment or part.
  • Calculate degrees: convert the percentage values for each segment into degrees relative to the 360 degrees in the circle.
  • Construct the chart: draw a circle and divide it into appropriately sized segments.
  • Add labels and a title: label each segment or add a legend to identify the segments. Then clearly title the chart.

Variants of the Pie Chart[edit]

There are different variants of pie charts [Wikipedia, 2008]:

  • Exploded Pie Chart: One ore more sectors are separated from the rest, to highlight it.
  • Perspective (3D) pie chart: Mainly for aesthetic reasons the pie chart is displayed in 3D. But the third dimension does not improve the reading of the data. On the contrary, these plots are difficult to interpret because of the distorted effect of perspective associated with the third dimension. In general use of superfluous dimensions not used to display the data of interest is discouraged for charts in general, not only for pie charts.
  • Polar area diagram: The polar area diagram is similar to a usual pie chart, except that the sectors are each of an equal angle and differ rather in how far each sector extends from the centre of the circle, enabling multiple comparisons on one diagram.
  • Multi-Level Pie Chart: Such charts are used for representing hierarchical data. The hierarchical structure of data is depicted by means of concentric circles.

Criticism on the Pie Chart[edit]

Comparison of Pie Chart vs. Bar Chart

While pie charts are common in business and journalism, they are uncommon in scientific literature.

Pie charts can be an effective way of displaying information in some cases, in particular if the intent is to compare the size of a slice with the whole pie, rather than comparing the slices among them.
[Wikipedia, 2008]


It has been also shown that comparison by angle was less accurate than comparison by length. Also a comparison by angle (as in pie charts) was shown to be perceived less accurate than comparison by length. The Usage of bars instead of slices improves the capability to compare the different segments of data [Wikipedia, 2008].

Other Forms of Charts[edit]

Among pie charts, used to display percentages, other forms of charts can be used for different purposes [French, 2008]:

  • Column Charts – are used to show comparisons between items of data. A column in the chart represents the value of one item of data.
  • Bar Charts - are very similar to column charts, except they run horizontally on the page instead of vertically like column charts.
  • Line Charts – are used to show trends over time. Each line in the graph shows the changes in the value of one item of data.

References[edit]

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