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
Unemployment in America, Mapped Over Time
Watch the regional changes across the country from 1990 to 2016.
Famous Movie Quotes as Charts
In celebration of their 100-year anniversary, the American Film Institute …
10 Best Data Visualization Projects of 2015
These are my picks for the best of 2015. As usual, they could easily appear in a different order on a different day, and there are projects not on the list that were also excellent.