At a Glance  Pie Charts/Circle Graphs
Pie Charts, also known as circle graphs, are ways of displaying the proportions, or percentages, of data that fall into different categories. It makes sense that these graphs are useful for displaying categorical data.
To make a pie chart we start with a circle, and cut it into slices like a pizza. Yeah, it would probably make more sense if we went with a "pie" analogy, but where's the fun in being predictable?
If half the data (50%) falls into one category, its corresponding slice will be half (50%) of the pizza. If oneeighth of the data falls into one category, its corresponding slice will be oneeighth of the pizza, and so on. Our advice is to not invite too much data to your house when you're throwing a party, so that you can keep most of the pizza for yourself.
Sample Problem
Thinking about backpacks helps us get to our happy place, so we'll do so once again. Suppose students had the following colors of backpacks:
red, blue, red, red, red, blue, green, green.
There are 8 backpacks total. Half the backpacks are red, one quarter are blue, and one quarter are green. We can represent this by the following circle graph:
In this graph, we labeled each individual slice of the graph with the category it represented. We could instead have a key that explains what each color means. It may not be necessary in this example, but sometimes you will have superduper thin slices, so it's a good idea to get some practice. If, on the other hand, one of your special skills is the ability to replicate classic works of art on the head of a pin, you may have no need for a key.
If you're making pie charts and you don't have different colored writing utensils handy, instead of making the pizza slices different colors you can make the shading of each slightly different, like this:
The important thing is to be able to tell the slices apart. If that means drawing a different Black Eyed Pea inside each one, so be it. Whatever works for you. Just make sure Fergie has the biggest slice. You know she'll get all "diva" about it.
Example 1
Use a pie chart to show the distribution of animals in a zoo that contains 40 penguins, 20 polar bears, 10 llamas, and 10 jaguars. Well, at least the penguins outnumber the jaguars, so they might have a fighting chance if all of them get loose. #yeahright 
Example 2
The pie chart shows the favorite pie flavors of the students in Kelly's class. Use the pie chart to determine if each statement is true or false. Not that it's essential to this exercise, but note the measure of the ratio of the circumference of this chart to its diameter. Also pi. Freaky.

Exercise 1
Draw a pie chart (circle graph) for the following set of data:
Mrs. Leahy has 4 cats, 4 dogs, and 4 fish.
Exercise 2
Draw a pie chart (circle graph) for the following set of data:
Of the sixty kids in Jimmy's class thirty ride the bus to school, twenty get rides from their parents, and 10 walk.
Exercise 3
Draw a pie chart (circle graph) for the following set of data:
A poll of Nancy's book club revealed that everyone liked chocolate. Four members preferred dark chocolate, and 6 members preferred milk chocolate. Maybe Nancy's book club should spend less time eating and more time reading. Just a thought.