When scanning a root drive that indicates the total number of files it contains, such as Macintosh HD, Disk Graph shows a completion percentage indicating the estimated number of files that still need to be scanned.
Sometimes the percentage can appear to change very slowly. This can happen when the scan encounters certain system directories that macOS, for some reason, isn’t able to traverse as fast as others.
Sometimes the percentage can appear to be stuck, particularly when it reaches 100%. This may be because Disk Graph encountered a larger number of files than was reported by the root drive, or because of the post-scan processing which may last a couple of seconds when the number of files is very large.
To check if Disk Graph is still scanning any files even when the progress seems stuck, you can open the detailed scan logs. Usually you can see the list of error logs by selecting the main menu Window > Error Logs. If you hold down the Option key, Error Logs changes to Detailed Logs, which will additionally list all scan logs. The scan logs will be discarded again as soon as you close the logs window.