CBQE-0: some improvements in plotter

Change-Id: Ie8eef1d49dfc32b32024692baba3dc3b11fc63e3
Reviewed-on: http://review.couchbase.org/27692
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>