The Functional Art: An Introduction To Information Graphics And Visualization / quote 10 of 18
First ask what users will do with your graphic
It is tempting to propose rock-solid rules—if you want to show change through time, use a time-series chart; if you need to compare, use a bar chart; or to display correlation, use a scatter-plot— because some of these rules make good common sense. [...] But reality is complex, and hard-and-fast rules can transform sound advice into immovable law. Exceptions and nuances can arise with the particularities of each project. What is really important is to remember that no matter how creative and innovative you wish to be in your graphics and visualizations, the first thing you must do, before you put a finger on the computer keyboard, is ask yourself what users are likely to try to do with your tool.