The error message is confusing when encountered during the plot of predicted vs. observed data fold lines. <img width="1472" height="312" alt="Image" src="https://github.com/user-attachments/assets/366218f3-3487-43d0-9ee1-2cb7478ccc8c" /> In this case, the independent axis is not time, but observation value. _Originally posted by @songsong02 in https://github.com/Open-Systems-Pharmacology/Forum/discussions/2000_