When you view the Grader Report for a large course which has quite a few users, who have completed quite a few assignments, the Grader Report will (obviously) get very large.

When it gets beyond the size of the screen, a horizontal scroll bar is added to the div containing the table.

However, if the number of users is also larger than the height of the screen, the horizontal scroll bar disappears well off the bottom of the screen.

Although on the whole, the interface needs a bit of an overhaul, an initial simple fix would be to limit the height of the div containing the grader report such that it doesn't disappear off the bottom of the screen, thus making the horizontal scroll bar accessible at all times.