rename "Use as abscissa" to "As X-axis"

Change-Id: Ic6e089417b684a41ab6578b9d0c4fde7fe1c72b6
Reviewed-on: http://review.couchbase.org/32575
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>