This is in 1.4.0. Though we've only tested one level of nesting.
Imagine a network plugin traffic that reports trafic by destination port, source port, and destination host. The resultant "headline" graph would be a graph showing "total network traffic in bytes" or some such. Click it and go to a page showing the traffic broken down by those three axes, identifying the Top 10 along each axis, for each interval space, in the legend with distinct colours.
Implementing "top 10" with rrd is a problem, the application has to pick the top 10 for rrd and generate the correct graphing command.
Imagine a network switch plugin generating a "headline" graph showing the overall traffic through the switch. Click it and get a page showing 1- the same graph with the ports in different colours, different health graphs (fans, power, temperature). Click the port graph and get to another page with each port in single graphs.
This last is something I want to be able to implement using the multigraph extention and using the namespace structure to show what to graph at each level and use as "headline" graph.
There is the original page with requirements and implementation considerations and a more implementer directed Multigraph example output page.