This usually happens when the dimensions or expressions have no label or the space is very limited. Anyway, there are two ways to work that around. The neat one is right clicking on that part of the chart, selecting "Custom Format Cell" from the drop down menu and set the colors and backgrounds to white, transparent, or what you need.
In order to see the "Custom Format Cell" option in the menu you will need to go to the Settings menu, User Preferences, Design tab, and click on "Always Show Design Items"
If this does not work, the not neat at all option is to create an empty text object with the size and colors you need and place one level upper the object (in the Layout tab of the object).
Hope that helps.