Pie charts are a means of visualizing percentages and data. They are different than other types of graphs because of the circular appearance and the use of percentages rather than raw data. Since the graph is different, there are some specific steps that must be taken to prepare the data and make the graph.
Using Proportions
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Unlike other charts, such as a bar graph or line graph, the pie chart relies on percentages rather than raw data. You need to convert the data to percentages for a pie chart. Divide the amount for each category by the overall total. For example, if you were making a pie chart of the number of students who majored in the different departments, you would have to convert the number of students to the percentage of the students. If there were 1,000 total students and 400 majored in science, divide 400 by 1,000 and get 40 percent as the percentage used in the pie chart.
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Determine Slice Size
Each category gets a slice of the pie chart proportional to the percentage of the whole. If creating the pie chart by hand, you will have to determine how many degrees of the circle each slice is. Since the circle has 360 degrees, multiply the percentage for each category by 360 to determine how big to make each slice. For example, multiply 40 percent by 360 degrees to get 144 degrees for the slice of the pie chart for science majors.
Drawing the Graph
Use a compass to draw the circle. Then draw a line from the top of the circle to the center of the circle. This will be the starting point for the first slice. Use a protractor to measure the degrees for the first slice and then draw a line to enclose the slice. For the science department slice, you would measure 144 degrees from the line and then close the slice. You would start the next slice from that line.
Labeling the Graph
Make sure that you label your pie chart clearly. Above the chart it is necessary to have a title that explains what data you are representing. For each slice, you can either label the slice or use colors or textures for the different slices so anyone viewing the chart will understand the data being shown. If you are coloring or using textures, make sure you include a key that explains what each color or texture signifies.
Uses for Pie Charts
Pie charts are useful in a number of different industries as well as for personal use. Governments and corporations must track their sources of revenues as well as the percentages of expenses different categories take up. Scientists measure the composition of water, soil and air in percentages. People break down their expenses by categories to determine where they are spending their money and how they can save more.