In the legend you can have two things:
1) in case of auras, the name of the auras
2) in case of the classNames, the names of the classes
if you toggle the "auras", it just turns on/off them
if you toggle the classNames, it should show/hide nodes that belong to the particular class (here is an example: http://jsfiddle.net/rech73ba/)
There is no click event on the aura names in legend though.
It is possible to build external control that would show/hide nodes based on auras, though.
Janis