What are the two signal parameters presented to the user on the waterfall display (spectrogram) of a modern receiver?

A waterfall display, or spectrogram, is a visual tool on modern software-defined radios (SDRs) and advanced transceivers that shows radio signals over time. The horizontal axis always represents the frequency, allowing you to see exactly where signals are located across the band. The second parameter is amplitude, or signal strength, which is represented by the color intensity or brightness of the marks on the screen. Stronger signals appear in brighter or warmer colors (like red or yellow), while weaker signals appear in cooler colors (like blue). Options mentioning phase, bandwidth, or digital modes are incorrect because a waterfall display's primary axes map directly to where a signal is (frequency) and how strong it is (amplitude), leaving the interpretation of mode or phase up to the operator.