How I would tackle this in broad lines:
SNMP access to your router
Get DHCP leases (there should be an OID for that, if I’m not mistaking)
Dump data to other environment
Use your graphing tool of choice to display data
OR …
Use Jotne’s SPLUNK solution to get a lot more out of it.
http://forum.mikrotik.com/t/tool-using-splunk-to-analyse-mikrotik-logs-4-0-graphing-everything/153043/1