That cron command doesn't do the statistics, I believe its the "plesk daily" script in /etc/cron.daily that updates the Plesk statistics (and if the Plesk statistics aren't working its possible that webstats aren't working).
Do you have a file named something like 050pleskdaily in /etc/cron.daily? Its plain text so you can try adding some debugging to it, but as I say I think it also does the webstats (I could be wrong on this) as well as general usage.
I had to go look for the graph you mentioned... never seen it before, we don't use plesk 10 much

Paul.