On the bottom, +1 is perfect phase correlation between the left and right. -1 is perfect 180 degree phase cancellation between left and right… In general on mixes, phase correlation is rarely perfect (unless someone mixed in mono on purpose) - hence the reason the lines move around a little… The further right, the more “mono” the mix… Further left, the more “stereo” the mix.
I appreciate the help here toader, and, as you describe, using the Stereo Expander plugin does indeed move the green line, the far left position producing a mono mix. But the last line of this paragraph above: “The further right, the more “mono” the mix… Further left, the more “stereo” the mix.” This seems a typo?