Scatter Diagram. The visual nature of the scatter diagram makes it a shoe-in for use on my Desert Island. Not only does the shape of the scatter illustrate relationship strength and direction, but it allows for the identification of outliers. Outliers can be the most informative part of a data set. If you understand why they occur, you may discover important guiding principles about the system from which the outlier originated.
A good graph should raise as many questions as it answers. The scatter diagram is the first step in developing a predictive model, and can reveal if that model works over a limited or extensive range of the predictor or outcome variable.