In my previous article, Meet D3: Data Driven Documents, I discussed my experience learning D3, in particular, discovering what D3 is not: it is not, strictly speaking, a SVG library. You will not find a function in the library to draw a circle or rectangle. Instead, you use its powerful selections interface to manipulate the DOM.
For example, assuming the DOM contains this element:
If we want to add a green circle with radius 10px at point (100,100), there is no draw-a-circle function in D3. Instead we do this: