This is no problem for Zenoss, what you need to do is modify your ethernet Monitoring Templates.
- Click "Advanced"
- Now "Monitoring Templates"
- Now select /Device under one of the ethernet templates tree
- In the Graph Definitions panel select your thoughput graph
- Now click the "Gear" and select "Custom Graph Definition
Under here you can add something similar to my 95th Percentile defintion below.
VDEF:in95=ifHCInOctets,95,PERCENT
VDEF:out95=ifHCOutOctets,95,PERCENT
COMMENT:\s
COMMENT:95th Percentile
HRULE:in95#00ff00:In
GPRINT:in95:\:%0.2lf%Sbps
HRULE:out95#ff00ff:Out
GPRINT:out95:\:%0.2lf%Sbps\l
Oh and if you have a few thousand devices like I do, be prepared for this to hose your ZODB for an hour or so.
Easy Right? 
Cheers,
Rod.