6.1.3 How do I draw a pie chart?
You must have data for which you need to show the proportion of each category as a part of the whole. Then the process is as below.
Collect the data so the number per category can be counted.
In other words, decide on the data that you wish to represent and collect it all together in a format that shows shares of the whole.
Decide on a clear title.
The title should be a brief description of the data that you wish to show. For example, if you wish to show market share of brands of washing powder, you could call the pie chart ‘Percentage market share of named brands of washing powder in the UK in (date)’.
Decide on the total number of categories.
In other words, identify the categories that you will use. For example, with washing powder, there may be dozens of brands on the market, but most of the share of the market will be held by, say, half a dozen brands. Therefore, you might decide on those six along with a category called other. It depends very much on what you are trying to show, but pie charts are likely to be clearest with no more than eight categories.
Calculate the degree share in each category.
First, add up the total number of instances that you have. For example, in the case of washing powders, the total number of instances might be the number of units sold in a year, so the share of the whole for each category would be the number of units sold in that category. To calculate degree share, you need to divide the share that the category has by the total units and then multiply by 360 (there are 360 degrees (°) in a circle).
So, if there are 200 units in all, and:
Make a final check that the number of degrees adds up to 360, then draw the pie chart.