Currently we only display unique visitors per day. We could run the query to also process this number across all logs for weeks and month. This might interfere with log purging, eg. if we purge logs that are older than 2 weeks, we are not able to process uniques for the month anymore.
I attach the small patch that solves the problem.
You need to keep at least one month history of piwik_log_visit in order this function can return proper results for month period stats.
Maciej, we can commit your patch to core - can you just please replace == 3 and == 2 b the real constant; looks good to me
(In ) fixes #842 computing the real number of unique visitors per month and week