Faceting

One general visualization technique that will be useful for you to know about to handle plots of two or more variables is faceting. In faceting, the data is divided into disjoint subsets, most often by different levels of a categorical variable. For each of these subsets of the data, the same plot type is rendered on other variables. Faceting is a way of comparing distributions or relationships across levels of additional variables, especially when there are three or more variables of interest overall. While faceting is most useful in multivariate visualization, it is still valuable to introduce the technique here in our discussion of bivariate plots.

For example, rather than depicting the relationship between one numeric variable and one categorical variable using a violin plot or box plot, we could use faceting to look at a histogram of the numeric variable for subsets of the data divided by categorical variable levels. Seaborn's FacetGrid class facilitates the creation of faceted plots. There are two steps involved in creating a faceted plot. First, we need to create an instance of the FacetGrid object and specify the feature we want to facet by (vehicle class, "VClass" in our example). Then we use the map method on the FacetGrid object to specify the plot type and variable(s) that will be plotted in each subset (in this case, the histogram on combined fuel efficiency "comb").