Our code release
http://www.chiefdelphi.com/forums/sh....php?p=1516461 may be helpful. Look at the bits about 'data logging'.
We logged the data to a file (in CSV format) on the roboRIO (either in /home/lvuser, or on a thumb drive plugged into the roboRIO). We could then copy the file off with SFTP/SCP/FTP (or pull the thumb drive), and plot it with Excel, Veusz, or something else. It's not realtime, but that wasn't our intent.
We *have* done realtime plotting with a custom network tables client on the driver station, it's doable, but icky.
It's written in Java, but it should have enough detail in there to get you going (writing files in C++ isn't any harder than in Java). PM me if questions.