Which packages are skipped, and for what reasons?

This is a little bit harder.

As initially written, a default buildenv was assumed so as to make the database updates more efficient (currently i386-6).

Most of the Makevars (PORTNAME, PORTVERSION, RESTRICTED, and so forth) do not (or should not!) vary by buildenv. The ones that most often do are:

Therefore, we either need some help from pointyhat to tell us why each package was skipped, or we can try to evaluate it at the time we build the charts and graphs.

In general, BROKEN ports are not built, but this can be forced. IGNOREd ports are never built.