Scatterplots of all pairs of variables
The problem of displaying relationships becomes even more difficult when there are more than three variables. It is possible to gain some insight into their relationships with an array of scatterplots of all pairs of variables, called a scatterplot matrix.
Brushing
Although a static scatterplot matrix reveals some aspects of the relationships between the variables, more insight into the data is obtained by adding dynamic features.
On a computer display, the scatterplots can be dynamically linked, so that clicking on a cross on one scatterplot highlights that individual in all scatterplots. Brushing extends this to allow highlighting of multiple crosses on a scatterplot with a 'brush' tool.