The chart values can be displayed in megabytes or as a percentage of disk space. Use the Log File Space list to change the values displayed on the chart.
The log files displayed can be restricted by clicking the database(s) of interest in the Databases grid. They can then be further restricted by clicking just the log files of interest in the Log Files grid. Hold down the Shift key to select a block of multiple records or the CTRL key to select individual records.