Sometimes, having either a more detailed view of both Time and BPM during a session is helpful. Conversely, it's possible that it's too much information and that longer ranges are preferable.
To manually adjust the Heart Rate/Time axis, select the View tab - drop menu and select, Show HRV Scale.
By checking the box to Enable Manual Controls, you may then make your own adjustments.
NOTE: With the box deselected, the program defaults are used.