The dimension (or index) of the data hyper-cube which
is not to be used as loop-index when plotting all the
selected data. Usually corresponds to X-axis selection,
i.e. it is "pt" for "Time", or "ch" for channel/wavelength,
etc. If plotting visibilities vs channel index, for example,
selecting "ch" as slice would produce one plot per scan,
unless the "All in 1" option is selected.