[FlowingData](http://flowingdata.com/) has a great [post about learning the fundamentals of graphical perception](http://flowingdata.com/2010/03/20/graphical-perception-learn-the-fundamentals-first/). In particular, the common techniques for encoding quantitative information are ordered by how accurately people can decode them according to William S. Cleveland and Robert McGill’s paper [Graphical Perception: Theory, Experimentation, and Application to the Development of Graphical Methods](https://secure.cs.uvic.ca/twiki/pub/Research/Chisel/ComputationalAestheticsProject/cleveland.pdf). For a lot more information about this type of thing, see Colin Ware's [Information Visualization](http://www.amazon.com/Information-Visualization-Second-Interactive-Technologies/dp/1558608192/ref=sr_1_1?ie=UTF8&s=books&qid=1269279751&sr=8-1).