Although no changes are made to piwik application, Unique Visitor Records are being dropped by more than 50% in piwik_log_visit table since 1st october 2014.
Backend mySql Query:
date(a.visit_first_action_time) as Visitday
,count(distinct custom_var_v1) as DistinctVisitor
FROM piwik413.piwik_log_visit a
Group by date(a.visit_first_action_time)
Order By date(a.visit_first_action_time)
just by looking at the data you provided, I'd say this is not a PIWIK-related issue.
If you have not updated or changed anything about PIWIK, then the reason must be elsewhere,
and looking at the quite strange bumps in visitors (hey, it ranges from 51 to 780 before october 1st), I'd say there has been some kind of trouble going on before...
maybe google picked up on that and is sending you fewer visitors since october 1st? There can be a ton of reasons... no way of telling like this, sorry.
Please ask in the forums as maybe it's related to your setup somehow? http://forum.piwik.org/ cheers