Oops! There is no tutorial on transitions yet. Sorry, I realize that was misleading. But I wanted to tell you about two great resources:
First, check out my Eyeo Festival talk on D3 transitions. It’s interactive, silly, and fun. Better than all of the tutorials so far put together. Maybe.
Second, check out my book, which has a whole chapter on transitions, and is actually somewhat readable.
These tutorials have been expanded into a book, Interactive Data Visualization for the Web, published by O’Reilly in March 2013. Purchase the ebook and print editions from O’Reilly. A free, online version includes interactive examples. Download the sample code files and sign up to receive updates by email.
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These tutorials have been generously translated to Catalan (Català) by Joan Prim, Chinese (简体中文) by Wentao Wang, French (Français) by Sylvain Kieffer, Japanese (日本語版) by Hideharu Sakai, Russian (русский) by Sergey Ivanov, and Spanish (Español) by Gabriel Coch.
Also see my video courses, “An Introduction to d3.js: From Scattered to Scatterplot” and “Intermediate d3.js: Charts, Layouts, and Maps.”