The Reference axis option in the Global Options dialog box can only be used when your graphs have a reference axis, and this axis is not scaled by a time line. (Reference axes are defined in the introduction to this section.)
This option lets you assign the reference axis to the vertical or horizontal axis of the diagram. Clicking on X-axis scales the reference axis along the horizontal axis. This is the default.
Clicking on Y-axis scales the reference axis along the vertical axis. The x- and y-values for the curves points are also reversed. As a result, the original graph appears turned on its side.
A reference axis cannot be scaled logarithmically.