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This (rather old) paper is still worth reading: Cleveland, W. S. (1984), "Graphical Perception and Graphical Methods for Analyzing Scientific Data"

I've read books by both these authors and they were very good. Their websites cover most of the important content found in the books.

http://www.nbr-graphs.com/ got some example. Her book was also a good and easy read on understanding more into the topic.

See the codes or products of javascript library d3 http://d3js.org/ I learned a lot from here