Lansweeper does not currently keep a history of free disk space. You can enable history tracking for scan items under Configuration\Item Wait Time (Lansweeper 5.3) or Scanning\Scanned Item Interval (Lansweeper 6.0), but you'll notice that the History checkbox cannot currently be ticked for the HARDDISK item responsible for free disk space scanning. This feature is on our customer wish list, but we do not have a release date for it at this time.
To manually keep a history, you could have a free disk space report sent to you via email on a scheduled basis. Information on setting up email alerts can be found in
this article.