What are those bars in audio visualizer exactly? Need to make visualizer with help of ffmpeg

Question

I'm trying to understand what are those bars in audio visualizers exactly.

I tried to decode a mp3 file using ffmpeg like this:

ffmpeg -i music.mp3 -t 1 -f s8 -acodec pcm_s8 -ar 1 out_1sec_sr1.raw

The output of this command is raw data of first second of the music.mp3 in 1hz samplerate. But the output file contains just 2 bytes... one for the left channel and one for the right channel? I'm pretty sure that it is not possible to visualize a second of a music using only two bytes? I guess each bar in below picture is at least one byte. Am I missing something in ffmpeg command? Is pcm_s8 codec appropriate for getting such data to visualize audio at all?

I'm using this library in my android project to use ffmpeg: https://github.com/brarcher/ffmpeg-android-java

I do not have any problem in drawing those bars and reading raw data file from storage using inputStream.