What is a comparative pie chart?

A pie chart is a chart in the shape of a pie. Draw a circle. From the centre draw 'wedges' (as you would cut a pie). Each wedge represents a statistic. For example in a wood, 33% of the trees are oak, 27% are ash, and the remaining 40% are elm could be represented in a pie chart where the largest piece of pie/wedge would be the 40%/elms, smallest - ash/27%. Thus you can see the comparison at a glance. This is a visual thing, not easy to describe in words. Surely you have a revision guide or text book to refer to Or try googling, to see if you can a diagram. Or try wikipedia, ditto.