"Layout and chart area" tab not editable?

EDIT: I am able to edit the settings under the “Layout and chart area” tab on the right, but only by typing in a new value. At first I expected to be able to delete or modify the existing values (hence the original post). All the text under that tab looks grayed out, as in this screenshot: https://docs.google.com/file/d/0B_p5Jk3qv53HeWlFaWo0VFh6aDg/edit?usp=sharing. I guess that was a hint that those values could not directly be edited, but it was confusing at first.