Step 2 (out of 6) from my blog on How to create a Flow diagram with a circular Twist in which a dummy arc (in light grey) has been added to visually separate the source of the flows and the target of the flows.
An example based on actual data can be found here
Modified http://d3js.org/d3.v3.js to a secure url
Modified http://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js to a secure url
https://d3js.org/d3.v3.js
https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js