'left main panel' only describes the location of that panel, not what it is set to display.
So it would be good if you gave some more info on that.
I'd like to be able to see Album Artists or Albums, etc. but them be able to sort them using other tags including Custom and Virtual Tags. For the time being it only allows for sorting by Count or Name which is a way too limited option, quite disappointing in fact.
As an example, I have a Custom Tag, "Album Composer", being a classical music aficionado. I'd love being able to use this "Album Composer" Custom Tag and then having those Album Composers sorted by another Custom Tag called "Album Composer Sort". Album Composer could be say, "Ludwig van Beethoven" and its sort value would then be "Beethoven, Ludwig van".