You can get pretty far with data graphics with just limited statistical knowledge, but if you want to take your skills, resume, and portfolio to the next level, you should learn standard data practices. Of all places, UK Parliament has some short and free guides to help you with basic statistical concepts. They provide 13 notes, each only two or three pages long that can help you with stuff like how to adjust for inflation, confidence intervals and statistical significance, or basic graph suggestions [pdf]. I like.
Statistical literacy guides for the basics
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Mapping the Spread of Obesity
A look at the rise for each state over three decades, for men and women.
Who Makes More Money
Someone mentioned that $400,000+ per year was commonplace in American households. That seemed like an odd comment.