CBQE-0: some fixes in plotter.py

Change-Id: Idba8a78846feded7df6280b4d2549b1c6611816e
Reviewed-on: http://review.couchbase.org/27485
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: Chisheng Hong <chisheng@couchbase.com>