You can use an oscilloscope to display the input and output of a circuit at the same time by:

A dual trace oscilloscope features two separate vertical input channels (usually labeled Channel 1 and Channel 2). This allows you to connect one probe to a circuit's input and another to its output, displaying both time-domain waveforms on the screen simultaneously over the same horizontal time base. This simultaneous display is the standard and most effective method for directly comparing a signal's phase, amplitude gain, and distortion as it passes through a circuit. Trying to map the input and output to the X and Y axes creates a Lissajous figure. While useful for comparing phase and frequency ratios, a Lissajous figure does not display the actual time-domain waveforms side-by-side, making the X/Y distractors incorrect for this specific purpose.