"The matrix chart divides the screen into a grid. Rows represent the values in one text column (e.g., product line) and columns represent another text column (e.g., country). Each cell then shows a circle or bar that represents the value for its row/column combination (e.g., returns for golfing equipment for Belgium).
Bars and Bubbles
The two modes of the matrix, circles and bars, are useful in different situations. Bars, whose height represents numeric values, are better for exact comparisons and allow space for more columns. Circles, which show values via area, are good for showing non-negative values that vary greatly, and allow space for more rows. You can flip between the two modes in the "Expert Options" menu to the left of the visualizations."