there are a couple of "plugins" which permit to create graphs from the data nagios collected (some require to install NDOutils).
When the RRD file is available it should be easy to export the data to a different interface (using links on the file system).
Personally i use nagiosgraph to create RRD files, but it has some pitfalls to be aware during installation and configuration (search the forum for threads about that). There are others but i haven't tried them..
Other solution for graphs: have a look at cacti. Easy to configure and install and lots of ready made plugins, could convince you to throw MRTG away and use only that one, sma eproblem you are facing, double interrogations to fetch data.