Function caption

pub fn caption(properties: &AudioProperties) -> String

The line drawn across the top of the picture.

The legend already carries the frequency axis; this says in words what the file claims to be, so that a picture kept on its own — sent to somebody, looked at a year later — still answers “at what sample rate?”.

The whole string is embedded in an ffmpeg drawtext expression, and part of it comes from the file itself, so it is restricted to a character set that cannot end the argument, start another filter, or escape the expression. See the test: this is the one function here with an attacker on the other side of it.