I did it for my
group window mod. It requires you create a progress bar style that has a vertical layout. All I did was duplicate the default layout and change the layout to vertical and rename it progressbarstyle_vertical (or something like that). That just tells the UI to draw progress in a different direction. If you want you can also mess with the Inverted (boolean) value which is another change you can make. I didn't try it but I assume you can get the bars going in the complete opposite directions that way (i.e. go right to left or up to down instead).