Y-axis of Standard Autocorrelation graph

Hi I’m writing a paper on determining the plucking position of a guitar string by analyzing the autocorrelation graphs on audacity. And I’m kind of confused what the axes are graphing. Is the y-axis the amplitude (C_n) and if so, is it in mm or cm or decibels? Thanks!

I’m not sure how the vertical axis is scaled, but the algorithm appears to be derived from this paper: https://labrosa.ee.columbia.edu/~dpwe/papers/ToloK2000-mupitch.pdf