Straight to the point. Pragmatic, no frills.
D3.js in Motion teaches you to transform your boring data into a variety of engaging and interactive graphs and charts. You'll start with an introduction to the world of D3 and data visualization by plotting simple data sets onto charts. In each lesson you'll dig deeper, turning your data into readable graphical representations like scatter plots, bar charts, line charts, pie charts, and more. Along the way, you'll discover how to create a fully interactive dashboard featuring multiple visualizations that pan, zoom, and interact with each other!
I don't know of anything close to this in free online resources.
A very good and very clear introduction to D3.js for beginners and intermediate level developers.
Explains concepts in a detail rarely found in blog posts or books and has a smooth progression from concept to concept. It will take you from zero to hero!