Where did you get those graphs? I've found my apache processes at 1.4 gig, and when apache is running memory usage goes from 1.9 gig to 8 gig (100% of memory).
I've severely reduced StartServers, MinSpareServers and MaxSpareServers but just 10 processes still consumes all my memory.
updated: found the graphs. Needed to install PHM (plesk health monitor).
But now see in health monitor that Real Memory Usage is pegged at 100% and Apache constantly growing using close to all the memory.