Each data file space consists of used and free space.
The chart values can be displayed in megabytes or as a percentage of disk space. Use the File Space list to change the values displayed on the chart.
The files displayed in the chart can be restricted by clicking on 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 files of interest in the Data Files 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.