I'm looking for a way to (in runtime) edit diagrams in a format much like UML. The catch being that I'd like to find a solution that works on all platforms. A GTK# control would fit that bill pretty well, while Winforms is a bit harder to work with. WPF is totally out of the picture.
A lot of solutions listed before seem to be woefully out of date. Any suggestions or recommendations that I perhaps missed? Also accepted are any answers that provide a 'starting point' for just rolling my own. I'm not incredibly familiar with GTK#.