Graphics (and numerics) for comparison

Most statistical data analysis, and thus most graphical data analysis, is directed towards modelling of relationships, but many statistical problems have a different flavour: their focus is comparison, and the key question is assessing agreement or disagreement between two or more data sets or subsets with variables measured in the same units. I survey the range of official and user-written graphical programs available in Stata 8 for such problems, with emphasis on making use of all the information in the data. Recurrent themes include (1) the use of reference lines, especially horizontal reference lines, indicating benchmark cases; (2) the relative merits of superimposition and juxtaposition; (3) how far methods work well at a range of sample sizes; ( 4) standing on giant's shoulders by writing wrappers around existing Stata commands; (5) use (and abuse) of summary statistics appropriate for such problems.