I want to draw a schematic process diagram in QGIS (yes, I really want, because I can document my data in the table) but it seems impossible to create a CRS that is simple Cartesian (x,y). Anyone can give me a hint?
You could use any projected CRS you like, they are all simple Cartesian grids, but don't use a geographic (lat/lon) CRS. You could use EPSG:27700 for example.
If you go to project properties, CRS tab you can turn on the "no projection" option. This should achieve what you're after.