I don't see why the author needs to add a "what is D3" section to a tutorial. A visitor to a tutorial site for library_x is pretty likely to know what the purpose of library_x is.
I think he's targeting his audience well. I'd imagine most of his users arrive like this...
1) I need a data visualization library.
2) D3 seems to be a good one.
3) Read the D3 docs at the project's homepage.
4) I still need help learning D3.
5) Google for tutorials.
6) End up at this person's tutorial.
"What is D3" would make sense if users did this in reverse (like I'm sure some people clicking on this news item have done), but I don't think that's the normal use case and I don't blame the author for not addressing it.
Disagree. I (thought) I know roughly what D3 is, but it would still be nice to have a (single paragraph, even) introduction. A framework can be pretty broad, so the intro would also signal what the author beliefs D3 is useful for and what techniques he intends to teach the audience.
For these reasons, I hold that any piece of technical writing needs an introduction, no matter how familiar the intended audience is. It doesn't need to be long. But it does need to have its place (at the beginning).
HOWEVER! Either we all overlooked this: http://code.hazzens.com/d3tut/lesson_0.html (because this article links straight to lesson one) or the author has added an introduction in the mean time, because it does have an introduction :) So, all is good :)
(BTW I thought that D3 also brought a 3D framework to the canvas, but I see no mention of that anywhere--maybe I"m confused with the name and something else?)
I think he's targeting his audience well. I'd imagine most of his users arrive like this...
"What is D3" would make sense if users did this in reverse (like I'm sure some people clicking on this news item have done), but I don't think that's the normal use case and I don't blame the author for not addressing it.