There is a problem with files that were deleted and reactivated. CVSPLOT would produce spurious events when the files were reactivated.
When using lines or linespoints to make plots, Gnuplot breaks lines if there is a gap in the sequence. If there are multiple users, each user's graph gets broken if any other user makes a commit between successive commits. This generally means you get vertical lines on the plots and few if any horizontal lines. My "fix" is ugly but works.