In reply to Ksharp.

Thanks for your comment. You are correct that when you are interested in ONE variable, a calendar heat map enables you to see the time dependence. However, I wanted to show the dependence of missing values across SEVERAL variables in one plot. This can be useful if two or more variables have a similar missing-value structure. I also wanted to present a method that works when the independent variable is not time.

Another visualization that works is a heat map of the longitudinal data in which you show the nonmissing values along with the missing values. I did not show that here because I wanted to emphasize the structure of only the missing values.