I was able to achieve this using the Express XY Graph VI. When you drop it in your code, you will see an XY Graph VI and another VI called Build XY Graph. Go into the properties for the Build XY Graph VI and uncheck 'Clear data on each call'. This should make your point leave a trail.
I've posted a working example in another thread:
http://www.chiefdelphi.com/forums/sh...ad.php?t=73971
I hope this is what you're looking for!
-Brian