Provide the ability to deselect selected references in the `reference map graph` view

[mkdjr]

Is your feature request related to a problem? Please describe.
I cannot find a way to deselect and item I've selected in the reference map graph view. I would like to be able to do this so that I can clear the selected reference panel at the top of the graph when I am no longer interested in learning about that reference.

Describe the solution you'd like
Provide the ability to deselect selected references in the reference map graph view.