A bubble chart displays three dimensions of data (where all three dimensions are numeric, as opposed to column or line charts, where one axis is numeric and the other is a category) in a two-dimensional format.
The horizontal and vertical axes represent two of the three dimensions, as in other charts. The size of the colored circle or bubble for each data point represents the third dimension.
You can use a bubble chart to display the data in a Report Definition report by selecting the chart type in the toolbar at the top of the Chart Editor.