Proportional line width, precise anchor points

I would like to reproduce this graph (or something like it) in OmniGraffle:

http://www.scientificcomputing.com/uploadedImages/Images/0907/sc090727h_lg%282%29.jpg

I'm not sure I can.  The biggest problem has to do with the proportional 
spacing of the lines and how they attach to the boxes.  While I have 
manually entered the lines thickness, I don't see any way to get them to 
attach to their end boxes so that their thicknesses is taken in account 
with their attachment.

For example, look at the "Coal" box on the left.  There is a fat line 
going to "Electricity Generation" and a thin line going to "Industrial". 
  Those two lines should be spaced on the "Coal" box so that the little 
line attaches under the big line.

Anchors would be a way to do this, but I don't see an method other than 
manually clicking the box to attach a magnet.  That's imprecise and 
would be a real headache when trying to do the "Electricity Generation" 
box, for instance.

Any thoughts?

-Sean

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Sean Ahern
Oak Ridge National Laboratory
865-241-3748
AIM: ornlsean