The data we got was only the NUMBER of pieces of trash, not the WEIGHT. This is the tricky part about graphs, and science in general. You have to be very careful about what you think is true, or assume, before drawing conclusions.
What if instead of counting the number of pieces of trash, the students weighed the trash? Would the data look different? Probably so. Metal generally weighs much more than the other materials, and cigarette butts weigh very little. How would this change the appearance of the graph? It's a good bet that cigarettes would have a much smaller pie piece, and metal would have a much bigger piece.
Now that you've learned all about pie graphs (piece of cake!), go on to learn about bar graphs and line graphs.