o Steps to add a statistic

Define an enum value in statistic.h, such as STATISTIC_FOO.

In statistic.c add a struct entry to to the statistic_name_map.

In preferences.c add .array[N] = STATISTIC_FOO to .display_stats.

In preferences.c add the symbolic name(s) for the statistic to stat_map.

In top.c you should add the type to the sort function, if needed.

Create or extend one of the .c files and add it to the Makefile.
You could start by copying user.c and user.h.

You should now be able to do: 
 -stats foo

The foo statistic will also be inserted as necessary, depending
on the ordering of the preferences .display_stat.array, and the
available space.