Adjusting the line thickness in a flow map

Discussion created by debhollon on Apr 30, 2012
This isn't the clearest map, but it does show what I'm wanting.  If you look in the center, you can see that the thickness of the lines narrows.  How was this done?  I've accomplished it using an additional field as a base for the symbology.  But, is there a tool for this or a way to automate it?