title: Equivalent of scrollTop in d3.js
body: <p>I want to implement some simple svg manipulation when the window has scrolled to a certain point – either when it has been scrolled by a set number of pixels or when a div or section reaches the top of the viewport.</p>
  
  <p>I'd initially thought it would be straightforward in jquery, but I had a lot of trouble applying smooth transitions to svg elements.</p>
  
  <p>I'm reasonably comfortable in d3, so I'm now hoping to use that. My trouble is that I can't find a way to determine how far the window has scrolled. Multiple SO and Google searches have turned up nothing I can use.</p>
  
  <p>How do I find how much the window has scrolled or where an element is relative to the viewport in d3?</p>
  
  <p>I should emphasise that the problem is that I need to animate svg elements. jQuery does this rather poorly, hence my need for d3.</p>