Cleveland starts with some basic principles of graph construction, looking at ways of making the data stand out, legends and captions, banking to 45 degrees, and scales. Some of this is quite low level, in places almost a checklist of instructions. ("Use a pair of scale lines for each variable. Make the data rectangle slightly smaller than the scale-line rectangle. Tick marks should point outward.") And in some ways people producing graphs these day don't need to worry so much about this, since the plotting routines in statistics software such as R try to "do the right thing" by default.
Software won't, however, tell you what kind of graph to use. The core of The Elements of Graphing Data is a survey of different graphical methods, describing the most useful kinds of graphs and when they are appropriate. These include logarithm scales, distributions and qq-plots, dot plots, different representations of time series, scatterplot matrices, and coplots, with the use of colour and "brushing" (dynamic graphs) also touched on. Some of this assumes a basic knowledge of statistics: Cleveland covers the importance of showing residuals, gives a fairly detailed explanation of how to fit a loess curve, and looks at ways of presenting statistical variation.
Finally, part three looks at graphical perception, presenting some of the theory and empirical science behind the earlier advice.
This is all clearly laid out and extensively illustrated, with interesting examples of both good and bad graphs. Some of this may be excessive — do we really need a full page example to illustrate that leaving the exponents off the labels on a log scale is a bad mistake? — but mostly it's helpful.
There's nothing startlingly novel in The Elements of Graphing Data, but it is an elegant presentation of important ideas, which it also helps to place in their broader context. It doesn't jump out and grab one the way some of the showier books on graphical presentation do, but is all the more compelling for its rigour. It is recommended for anyone whose work involves the graphical presentation of technical information.
Note: thanks to Sean Carmody for giving me a copy of The Elements of Graphing Data.