djmurphy420/ggplot2-transition-guide — comment on commit ae050eb3133ce559adc1488edb96231fb2b08081
File: ggplot2-0.9.0-BW.Rnw line 1353
By yihui:

Sounds great. This is the first practical example as I know which uses this conditional evaluation feature :)

One reminder is that you have to compile the two versions under different directories so their cache files will not clash with each other. See my first draft code.