Small multiples, or the science and art of combining graphs

Good graphics often exploit one simple graphical design that is repeated for different parts of the data, which Edward R. Tufte dubbed the use of small multiples. In Stata, small multiples are supported for different subsets of the data with by() or over() options of many graph commands; users can easily emulate this in their own programs by writing wrapper programs that call twoway or graph bar and its siblings. Otherwise, specific machinery offers repetition of a design for different variables, such as the (arguably much underused) graph matrix command. Users can always put together their own composite graphs by saving individual graphs and then combining them. I reviewed small multiples at the 2013 London meeting, and this presentation offers further modest automation of the same design repeated for different data. Three recent general programs allow small multiples in different ways. crossplot offers a simple student-friendly graph matrix for each y and each x variable specified, which is more general than a scatterplot matrix. combineplot is a command for combining univariate or bivariate plots for different variables. designplot is a command for showing univariate summaries that generalizes and rewrites grmeanby in graph dot style.