Here's a (brain) dead simple rule about pie charts: if you're using them to demonstrate a difference or change, so that you end up showing two, or more, pies,

Here's a (brain) dead simple rule about pie charts: if you're using them to demonstrate a difference or change, so that you end up showing two, or more, pies, and if the aggregate amount represented by each pie isn't equal, then you're up to no good. Even if you try to compensate by making one of the pies have a larger radius than the other this is bad, as area comparisons will always be under perceived. If the case distinctions have the same summed aggregate, and you're just showing how the apportioning of the groups have changed then you're ok. But then graphically why not just have two bar charts which are visually more easy to interpret?