If you can summarize disk usage (du -sk /*/* or the like) and dump it in a spreadsheet, it can be graphed. It all depends on how you want to categorize usage: putpose, ownership, or just space used. You need df to find available space, you need to decide if hard links counted more than once is a problem, you need to decide if you want to cost in bytes or pages. I believe du and find have options to stay on one device. Sometimes, it is informative to find really large files as well as subtree summaries, depending on your focus.