My recent SU reputation history:
- There is an entry for Jan 3, on which There were no net reputation changes on this day. No posts are listed for this day.
- There is no entry for Dec 29.
It is unclear why Jan 3 gets its own (empty) entry, while Dec 29 doesn't. IIRC there was a quickly reverted up vote on Jan 3 (not sure about the date, but it'd make sense in this context). It could have been easy to forget about this rep change or even miss it completely.
There is absolutely no (non-technical) reason to distinguish this way between "no votes" days and "reverted votes" days in the reputation history. Either make sure that all days get their entries, or that days without data are removed from the list.