Enable UI to show variable flow through the canvas

[_sean_otto]

For example, in the Properties tool (I say that generally because it could be in more than one place in that tool) all the user to enable to visibly see the path of the variable in the flow on the canvas.  As shown in the mock-up image below (as crude as it is, I think you get the idea).  This would allow you to trace the path forward and backwards in the flow, seeing where the variable entered and when it exited.  This could be something you can turn on and off like the grid option.