The chart values can be displayed in megabytes or as the number of rows. Use the Table Space list to change the values displayed on the chart.
The tables displayed can be restricted by clicking the database(s) of interest in the Databases grid at the top of the drilldown. They can then be further restricted by clicking just the tables of interest in the Tables grid in the middle of the drilldown. Hold down the Shift key to select a block of multiple records, or the CTRL key to select individual records.