Beginner's question about line length
 

I've just started using OmniGraffle Pro, and I'm frustrated. I'm trying to create a phylogenetic tree, which is essentially just a line graph with nodes connected at internodes. The lengths of lines matters, and I can't figure out how to set that attribute, let alone to lock it so that elements of the tree can be dragged around without changing line lengths. The obvious way to do this would seem to be using the menu item Edit/Insert Variable/Line Length, but that item is always greyed out, whether or not I have a line item selected. I've searched this forum and read the manual, and I still can't figure it out. I'm sure the answer is right under my nose, which is frustrating.

Check out Tutorial 3 in OmniGraffle-5-Manual.pdf.
Click the Text tool; it's the one with a letter 'A' on it. Then click it again so that it'll stay active instead of reverting back to the selection tool after you use it. Put the mouse pointer over the line so that the line glows, then Edit > Insert Variable > Line Length.