How easy is it to attach event handlers to nodes through the API? This way, some user-defined code can be written to react to the mouse interactions with the graph (for example, if you use the library to show the navigation graph of the site, double clicking on the node takes you to the page represented by the node)

No Way.

We need a read-only mode for that. At present we have editor handlers attached everywhere, even if the menu is not shown. Actually labels could easily be made tags (in svg) but they could get in the way when dragging.

Just to clarify, I was just recording some thoughts for future work. (In other words, adding an API for attaching/detaching custom event listeners is not a requirement for completing the project)