I too have been having problems with the Picolog graph not displaying when a certain number of values have been reached. The problem still occurs with the latest software update. A solution I have found is to use the recorder for recording the values, and run it in the background at all times.
When you want to view and analyse the data, use the Picolog viewer, open the current data file, and the graph will appear in its entirety (but obviously not refresh with the new values unless you reopen).
A useable solution, but doesn't change the fact that the problem should not be occuring.