Long-time reader Nick B. found this attractive flow chart.
The chart was produced by the Internet Monk blog. The data was culled from this report (PDF) by the Pew Forum.
The cross-hairs trumpet excitement but the reader is left without much. One could tell that the unaffiliated proportion (red) has more than doubled, mostly at the expense of Catholics (green); that most religions retain the vast majority of their faithful (at least by internal proportions); and that people of one or another faith move to one or another faith.
Yet, any of these high-level insights do not require a chart that contains data on movement between each pair of religions.
One smart thing about this chart is the inclusion of "unaffiliated / no religion", which completes the picture; otherwise, some previously faithful people would drop off the chart (literally).
The other smart thing is its self-sufficiency: none of the data is printed on the chart, and I doubt readers miss them.
Here, I attempt an alternative, which is a variant of the Web of Debt chart discussed here.
Note the economy of colors, lines, etc. I have chosen to use the number of people with a particular childhood faith as the base for all the percentages; other bases can be selected. For example, the unaffiliated has grown by 144% of the childhood base, with about half of that growth coming from previous Protestants; meanwhile, an exodus of Catholics has occurred. (PS. the data for other faiths being incomplete in the aforementioned report, I made up some of the data so as to finish the chart.)
If the line thickness is made proportional to the percentages, that would eliminate the need to have all those numbers on the chart.