You only get a straight line in the diagram while you are removing the "free" water. That is, your product is wet. And that assuming that you don't have anything dissolved in that water.
As soon as you start removing water that is associated with the solid, be it capillarity, hydrogen bonds, etc, the pressure start to fall, or the temperature to reach the same vapor pressure starts to rise.
As latexman said, you need experimental data on the specific material you are drying.
Edit: the book "Mass Transfer Operations" by Robert Treybal, in the chapter about Drying (one of the last chapters), has a very good discussion about the water equilibrium while drying. Is is a classic, so almost for sure in your college library.
Edited by Saml, 01 December 2022 - 08:12 AM.