Pie charts
Introduction
Pie charts differ from most other graphs because they do not feature a horizontal and vertical axis. Pie charts are, however, a useful way of showing information in a clear and interesting way. This chapter looks at interpreting and constructing pie charts.
What is a pie chart?
A pie chart (or pie graph) is a circular graph which is divided in such a way that it resembles the slices of pie. Pie charts are divided into a number of parts to show the composition of a whole. The length of the arc, the angle that the arms of each segment form and the area of each segment are all designed to be proportional to the data they represent. Since a pie chart is a composition of a whole, quantity pie charts are not as common as percentage pie charts. In percentage pie charts, the data is shown in the form of a percentage.
See image 1
Like all other graphs, pie charts should always have a title, source and unit of measurement. On some pie charts, each segment is labelled with a name and an amount. In the case of this information being unable to fit on the chart itself, colours are used to distinguish between the different segments of the pie chart. The name and quantity or percentage that the colours of each segment represent can be found in the legend. A legend is a table which explains the meanings of the colours (or symbols and abbreviations) used in the corresponding graph (or map). The legend will most likely be located beside the pie chart.
Below is an example of a quantity pie chart (Graph 1) and a percentage pie chart (Graph 2). Both of these charts use a legend to assist in explaining the information displayed in the pie chart.
Graph 1. Quantity pie graph. Source: Geodata Coastline Database, 1993.
Graph 2. Percentage pie graph. Source: Geodata Coastline Database, 1993.
How to construct a pie chart
Using the information in the table below, construct a pie chart which shows the language composition in Australia.
Source: The World Factbook, (2001 Australian Census).
|
Language |
English |
Chinese |
Italian |
Other |
Unspecified |
|
Percentage |
79.1 |
2.1 |
1.9 |
11.1 |
5.8 |
|
Degrees |
79.1 x 3.6 = 284.76o |
2.1 x 3.6 = 7.56o |
1.9 x 3.6 = 6.84o |
11.8 x 3.6 = 42.48o |
5.8 x 3.6 = 20.88o |
You will need a:
- pencil
- paper
- protractor
- drawing compass
- ruler
See image 2
Step 1. Use a compass to draw a circle. Hint: It is easier to mark out the segments in the pie chart if the radius is greater than three centimetres.
Step 2. Using a ruler, draw a straight line from the 12 o'clock position to the very centre of the circle.
Step 3. Convert (data) percentages to degrees by multiplying them by 3.6 (360 degrees in a circle). Using the protractor, align 0o with the line just drawn in the 12 o'clock position. Mark out the first segment.
Step 4. Align 0o on the line which marked the first segment and draw the next segment. Continue this process clockwise around the entire circle.
Step 5. Colour and label each slice with the corresponding language and percentage.
This is what your pie chart should look like.
Once students have become proficient in manually drawing a pie chart, they can practise constructing a computer-generated pie chart.
See image 3
