Usually it shows a two-dimensional graph of one or more electrical potential differences ( vertical axis ) . These are plotted as a function of time or of some other voltage ( horizontal axis ) . An oscilloscope ( also known as a scope , CRO , DSO or , an O-scope ) is a type of electronic test instrument that allows observation of constantly varying signal voltages , usually as a two-dimensional graph of one or more electrical potential differences using the vertical or ` Y ' axis , plotted as a function of time , ( horizontal or ` x ' axis ) .