I'm trying to write a graphical widget, and needing some extra information.
Look at new custom chart widgets which are written using D3 : http://marketplace.thingworx.com/Items/Custom%20Chart%20Widgets it will be a good starting point.
Can you answer any of my other questions?
I think that under standard TW Graph widgets there's Raphael.
About D3, if you look at Custom Chart Widget documentation you will find this text that may bring some light about where D3 it's/was used:
Retrieving data ...