Over the past few months I've become increasingly fond of Edward Tufte's Slopegraphs for data visualization. They aren't just good for comparing across time, but also across two categories.
If you aspire to be a data scientist, you’re really aspiring to be a data wrangler. You see, 80% of your working hours will be spent wrangling the data. That’s on average. On some projects, you will spend more than 100% of your “working” hours with your lasso. I hope you enjoy that sort of thing.