The chart values can be displayed in megabytes or as a percentage of disk space. Use the Transaction Log Space list to change the values displayed on the chart.
The transaction logs 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 transaction logs of interest in the Transaction Logs grid. Hold down the Shift key to select a block of multiple records, or the CTRL key to select individual records.