mouseAxis in GuiScrollCtrl

How do I trigger event.mouseAxis = 0 (x axis) for a GuiEvent. I see it is being used in GuiScrollCtrl::scrollByMouseWheel, but I can't figure out what x axis scrolling is. My only thought is that it is the tilt of the scroll wheel, but that doesn't scroll a scroll control horizontally, unless there is something I am missing. The scroll wheel works file for scrolling in the y axis but just not x.

Anyone know what is up with this? Is it just not finished or is there some needed setup?