Learn in the textbookCross-Tabulation and Advanced Visualization of Correlation
DefinitionA scatter plot matrix (散布図行列) places a scatter plot for each pair of variables in a grid that lists the variables along its rows and columns.
ExplanationEach cell holds a scatter plot that uses the values of its row variable and its column variable as the two axes. The diagonal cells pair a variable with itself, so they can hold the variable name or that variable's own distribution. Points rising to the upper right show a positive correlation and points falling to the lower right show a negative one, which you can confirm by eye.
AnalogyIt is like a seating chart in roster order with a photo of a pair of people pinned to every seat, so every possible pair can be seen at a glance.