Over the years, quite a few graph editors seem to have been written in Tcl/Tk (the canvas probably has something to do with this). This page is for listing such applications and related material.
All of the above are about graphs in the Graph Theory sense, i.e., structures with a base set of objects called vertices (also nodes, points, …), and how these are connected by edges (also arcs, lines, …). There is also a concept of graph as in "graph of function", which is a form of data visualization — plotting various charts is at times referred to as graphing a representation of a set of data — but those graphs typically don't need to be edited by hand (deciding how they should look is more like configuring). If it is the latter kind of graph you're interested in, then you should probably rather have a look at Category Plotting .
Lars H remembers downloading a graph editor of French origin once, but cannot recall its name, only that it came as a tclkit. (It's possibly the app editgraphe described on http://wfr.tcl.tk/208 , but the download link there doesn't work anymore.)
IGD (Interactive Graph Drawing) is a graphical application and library of Tcl/Tk functions for displaying, manipulating, scaling and printing undirected graphs.