treat NaN as zero value in nvd3 charts

Change-Id: I2a1c5db76c7979a3218f77be893c3d4508b6d030
Reviewed-on: http://review.couchbase.org/30752
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>