wrong data displayed with postfix application

I guess there is something wrong with the postfix_mailgraph app in observium. The output at observium (attachment: graph_observium.png) is not the same than it is with the cgi-script from mailgraph (attachment: mailgraph.png).
I guess it has something to do with the update-cycle: Mailgraph updates its rrd-database every 60seconds per default:

D-MX01:/var/lib/mailgraph # rrdtool lastupdate mailgraph.rrd

 sent recv bounced rejected

1384885980: 0 0 0 11

D-MX01:/var/lib/mailgraph # rrdtool lastupdate mailgraph.rrd

 sent recv bounced rejected

1384886040: 0 0 0 12

and postfix_mailgraph is called from the unix_agent every 300sec... so only every fifth "lastupdate" is transfered to observium.