While I like yEdit, I think that http://www.graphviz.org/ would be the perfect answer, as @Izzy mentions in his comment.
I have been using it for years. It is excellent for drawing any diagram with nodes (trees, network diagrams, sate machines, etc, etc, etc)
Just take a look at the gallery.
It is very stable, very widely used, and well supported. It is also free and cross platform (you didn't state your o/s).
As stated in my well received previous answers recommending the tool, here, here, and here, you write a simple text file, then run it though the tool to draw diagrams of various sorts. There are a plethora of options to configure the layout of the digram which is produced.
Here is a very simple example taken from the examples page of the site:
a -- b;
b -- c;
a -- c;
d -- c;
e -- c;
e -- a;
I am sure that you can see how this can meet your needs. Just tweak it for line colour, shape of nodes, etc