Dimensionless axis plots
Lack of a unit never implies that a clear label isn't necessary.  How can time not have an appropriate, non misleading label?  How about "Index" or "sample number"?  Just because it doesn't have a unit doesn't mean that you can't come up with an appropriate label?  Even "Normalized Amplitude" or something.  If the lack of a  unit isn't immediately apparent, sometimes "unitless" where you would normally include a unit will clear things up.

Lastly, look toward your literature.  If you can't find an example close to what you're doing, you may be reading the wrong literature, or not reading enough of the right literature.  Having to ask about something like this is somewhat of a red flag on your due diligence.