to see if it is working go to your RRD directory and see if an modbus.rrd was created - /tmp/rrd/ "hostname" -- but it needs input so set it up and see what happens - you can also see if it setup collectd config file nano /etc/collectd.conf and see if the settings are there.. if not then see if they are being written to /etc/config/luci_statistics and if they are all written correctly and still no rrd file just run collectd manually and see what error it is throwing up for you
to draw graphs you will need to create a definition file
/usr/lib/lua/luci/statistics/rrdtool/definitions/
see my mqtt or snmp example on that
lately I have not used RRD graphing but instead got statistic to upload data to influxdb and used chronogaf or grafana to draw my graphs so I am currently a little weak on knowledge to tell you how to do it exactly to create the definition file