用R表示纵向分类数据的好方法

[Update: Although I've accepted an answer, please add another answer if you have additional visualization ideas (whether in R or another language/program). Texts on categorical data analysis don't seem to say much about visualizing longitudinal data, while texts on longitudinal data analysis don't seem to say much about visualizing within-subject changes over time in category membership. Having more answers to this question will make it a better resource on an issue that doesn't get much coverage in standard references.] Although I've accepted an answer, plea