monitorixにおけるnginxのステータスの追加は、/etc/monitorix/monitorix.confのnginxのステータスをnからyに変更すれば表示されます。
<graph_enable> system = y kern = y proc = y hptemp = n lmsens = n nvidia = n disk = n fs = y zfs = n du = n net = y netstat = y libvirt = n process = y serv = y mail = n port = y user = y ftp = n apache = y nginx = y //ここです lighttpd = n mysql = y varnish = n pagespeed = n squid = n nfss = n nfsc = n bind = n ntp = n fail2ban = n icecast = n raspberrypi = n phpapc = n memcached = n apcupsd = n nut = n wowza = n int = y verlihub = n </graph_enable>