Why did they put a non-linear x-axis on that graph? From 0% to 80% of income is only the first 40% of x-axis space! Then tick marks (with equal physical spacing) change from 5% to 1% increments.
We're talking about the top 90% of earners so it makes sense to enlarge the right side of the graph. They're trying to illustrate the huge changes in income for every percentage point. A linear axis would be basically vertical at the right end.
I think a vertical line would show the point more clearly. The idea is that there is a lot more change at the top end than the rest, and a vertical line does that.
Resolution does become an issue (I agree that showing the change from 99.5 to 99.9 is good and a linear scale makes seeing those points harder). A way around that is to have the graph scroll left to right -- which should be easy given that it is an interactive JS/flash graph widget.
Based on the way the info box follows the mouse, I think it's because their graphing tool puts an equal amount of space between data points.
Most likely, the data points they fed into it have x-values of [5%, 10%, ..., 75%, 80%, 81%, ...]. This is a useful way of tabulating percentile data, though not such a useful way of graphing it.
