dreamt I was in class again and someone was presenting a graph of year-over-year percentage changes in something. I said that was bad UI, because 1) -50% followed by +50% does not equal no change, and 2) to track changes, e.g. -10% down for month M in year Y followed by -10% down in month M in year Y+1, means you have to compound. Both of these imply putting a cognitive burden on the graph viewer as they play the graph from their own points of reference in time.
Some guy hotly contested this, so I had to walk through a demonstration of why it sucked. Three-quarters of the way through the demo, he figured it out and shut up.