The graph prototype belonging to the discovery rule 'Mounted Filesystem Discovery', which presents a pie chart about the space on each filesystem discovered has a very strange content. It consists of two parts of the pie: Free space and total space. This doesn't make any sense, since they are not complementary or mutually exclusive (everything is a part of the total space... obviously). It looks like the 'graph sum' type on the pie-piece type takes care of effectively making the Total Space piece actually represent the Used Space (Although neither the documentation nor the internet at large so far as I could find described the function of 'graph sum')