Modern waveform monitors usually have a feature that does exactly this. They replaced the older analog waveform monitors, which achieved a visually similar result but through rather different means. Very useful for video QC and calibration (I worked a few years as tech at a company that did exactly this and also had some analog ones to play with).

Some digital cameras have this as an overlay on the LCD. If you have a Canon EOS you can always get this feature via the really cool Magic Lantern open-source firmware.

Also, re: some comments calling it a histogram here: maybe colloquially (though I've never heard it), but generally it's called a waveform monitor.