Those principles espoused by Edward Tufte in The Visual Display of Quantitative Information (first edition, 1983) that I found helpful:
Excellence in statistical graphics consists of complex ideas communicated with clarity, precision and efficiency. Graphical displays should:
- show the data
- induce the viewer to think about the substance rather than about methodology, graphic design, the technology of graphic production, or something else
- avoid distorting what the data have to say
- present many numbers in a small space
- make large data sets coherent
- encourage the eye to compare different pieces of data
- reveal the data at several levels of detail, from a broad overview to the fine structure
- serve a reasonably clear purpose: description, exploration, tabulation or decoration (p.13)
Graphical excellence is that which gives to the viewer the greatest number of ideas in the shortest time with the least ink in the smallest space. (p.51)
The representation of numbers, as physically measured on the surface of the graphic itself, should be directly proportional to the numerical quantities represented.
Clear, detailed and thorough labeling should be used to defeat graphical distortion and ambiguity. Write out explanations of the data on the graphic itself. Label important events in the data. (p.56)
The number of information-carrying (variable) dimensions depicted should not exceed the number of dimensions in the data. [i.e. avoid pie charts] (p.71).
Graphics must not quote data out of context. (p.75)
Above all else show the data. (p.92)
Maximise the data-ink ratio, within reason.
Erase non-data-ink, within reason. (p.96)
Erase redundant data-ink, within reason. (p.100)
Forgo chartjunk [e.g. heavy grids and fake perspective] (p.121)
Mobilise every graphical element, perhaps several times over, to show the data. [e.g. use the axes of a scatter plot to show the range of the data] (p.140)
Graphics can be shrunk way down. (p.169)