There already exists a feature to enable you to do this!
Using the directive
routers.cgi*Extension[targetname]:
you can define a new link to appear at the bottom of the page - it doesnt appear as a button, but as a text link immediately above the buttons. This will let you link to any URL you choose, in the same or a new frame.
However, if you want to link to a separate routers.cgi graph, you should instead use the
routers.cgi*Link[targetname]:
directive. This lets you specify a separate cfg file and targetname to link to within routers.cgi, with link text if you want.
For an example of this, you can see the doc/EXAMPLES, or the ourput from the buildwan.pl script (it uses this to link the interfaces at different ends of a WAN link).
You can see the doc/MRTG_INFO file for the syntax of this directive.
Steve