modify4gn: interactive modifications of a gn
In g4n: graph for networks

graphical interface to modify (i) the positions, (ii) the arcs and (iii) the nodes of an object of class gn. Besides, it provides the facility to change the viewpoint from which is drawn the graph (positions of a gn object are defined in R to the 3).
The modified gn is returned.
To exit, just click the grey button 'E'
Action are started by the left hand buttons. Their codes are:
"rst" for resetting (cancel all modifications),
"MN" for moving the position of a node (the user must click twice: for indicating the node and then the new desired position),
"AA" for adding an arc (the user must click twice to indicate the starting node then the ending node),
"RA" for removing an existing arc (the user must click twice to indicate the arc to be removed),
"AN" for adding a node (the user must click once to indicate the position of the new node), its name is automatically provided but can be changed afterwards,
"RN" for removing a node (the user must click once, on the node to be removed),
"+" for narrowing the zoom (the user must click once, on the point where the zoom must be centered; the zooming coefficient is driven by the @pgr@kzoom parameter),
"-" for widing the zoom (the user must click once, on the point where the zoom must be centered; the zooming coefficient is driven by the @pgr@kzoom parameter),
"Rot" for rotating the representation in R to the three (the user must click once, in the direction of the rotation and more or less close to the center to indicate the angle to perform; the two big circles which appears on the diagram are associated to rotations of 5 and 10 degrees.