Nicer exponent formatting

[liamtoney]

This makes the exponent more clear by formatting it more nicely and placing it within the waveform axes (motivated in part by #8).

Before:

After:

[liamtoney]

Now the image shown in https://github.com/liamtoney/sonify/issues/8#issuecomment-1049053698 looks like:

This makes the fact that the exponent is referring to the waveform axis more clear.