Why doesn’t the cumulative percentage line start at zero? How do you do the calculations so that it does?

Pareto charts often include a P-P (Percent-Percent) plot such as Jon’s red curves. This represents a cumulative distribution of effect (# of apps) against the cumulative ordered distribution of categories (for Pareto).

It’s not at all ridiculous to show that such a curve passes through the (0,0) origin as well as the (100%,100%) point. In fact, at the risk of showing the obvious, that’s how I’d prefer to draw it. For example, QIMacros draws its Pareto chart in Excel that way, although it doesn’t explicitly put cumulative % on the category count as Jon finally did under duress.

If you check Wikipedia, you’ll find a distinction between P-P and Q-Q plots, and it is P-P plots that go through (0,0) and (1,1) at their edges, while Quantile-Quantile plots may go to infinity (for continuous variables) at their hypothetical vanishing edges (not generally plotted!).