Hi.
It's not possible to manipulate the Do Not Fragment bit on outgoing PingPlotter packets, which means PingPlotter can't explicitly show that.
You could certainly use the Ping command along with the route discovered by PingPlotter to see where you start getting a return ping. This is a bit imperfect as some routers don't respond to ICMP echo requests explicitly targetted at them, but that happens rarely enough that I'd suspect using a combination approach would be pretty effective.
Sorry we don't have a better answer for you (and also sorry about the long response time - somehow this question slipped past the "new message posted" notifier).
- Pete