Having selected a specific date upon which to view logged data (see Selecting a Date) and an appropriate time duration (see Changing the Time Duration Displayed on the Graph), you may use the scroll bar at the bottom of the Historical Data Viewer page to move backward and forward in time, in the increments you set using the duration drop-down list.
For example, if you've used the Select Start Date button to set November 1, 2003, and have set the duration drop-down list to 6 Hours, the graph will initially display 6-hours worth of data beginning at the current time on November 1st. As you click the backward or forward scroll bar button, the graph will adjust to display earlier or later data in 6-hour increments.