Juice Analytics put together a practical guide to build context on the topic of data visualization. It is quite handy and I would suggest at least running through it if you have an interest in the subject. This link will take you to a pdf copy of the guide.
What is most interesting to me is the format itself. By structuring online resources, integrating multiple media formats, and creating assignments, the guide is essentially a short course on practical data visualization. While many online guides utilize multiple sources they tend to synthesize the information rather than rely on numerous individual posts. Other times laundry lists of resources are listed with relatively little structure, which can be overwhelming for someone new to the subject. I hope to see more practical courses/guides developed using this structured approach.