If you want to learn visualization, you should learn data. To learn data, you should learn statistics. Where to begin? The free analysis courses offered on Coursera, by Johns Hopkins professors is probably a good place to start. Currently available: Computing for Data Analysis with biostatistics professor Roger D. Peng and Data Analysis with Jeff Leek, also a biostatistics professor.
There’s also a handful of data-related courses from other university professors that might be worth a look.