Grouped bubble charts are multiple series of regular bubbles. Bubbles can be seen as (x, y, z) points displayed on a cartesian plot area, where (x, y) are the 2D coordinates, and z is each bubble's size. Bubbles are usually normalized, i.e. limited to sizes between a minimum and maximum value.
Our US and France Sales demo charts show two bubble series, each with a different color: for sales in US and sales in France. There is a bubble at the intersection of each year value between 2000 and 2006 and the number of products sold in US or France in that year. The horizontal X baseline appears here as a category axis, with regular intervals for each year value. But bubble charts use also often a linear baseline, with X values distributed at random intervals. Main difference when compared to scatter charts is each bubble gets a different size. This is the third dimension, as an additional measure. In this case, some products may be more "valuable" than others, when same quantity has been sold.