This is an example of a tree digram where the nodes can be varied amongst the (range of symbols)[https://github.com/mbostock/d3/wiki/SVG-Shapes#symbol] available via the
Many thanks to Josiah who asked the question of how this could be done.
There are of course several different methods. A separate custom svg path could be declared and used or a more elegant and extensible method might be to apply the symbol type depending on the range of input values (as is used in Mike Bostock's Force-directed Symbols.
It is used as an example and described in the book D3 Tips and Tricks.