What is a multicast graph?
Here is a technical paper on the LMAX Architecture (I just followed a link found on the blog you linked here): http://disruptor.googlecode.com/files/Disruptor-1.0.pdf

An example of a multicast graph is on page 9 of the PDF. There are also examples of multicast graphs (compared to various other options) here: http://en.wikipedia.org/wiki/Multicast