This is the illustration of the "nested grids" what this utility uses for displaying the colored bars:
As you can see 1) Every single grid has two columns only 2) and contains rows as many as the childitems. 3) The column widths are constant. 4) The row heights are in proportion to the folder/file sizes. 5) The column 0 of a grid contains colored bars only. 6) The column 1 contains subgrids only.
The above picture displays the following hierarchy: