plotXYsurface undocumented
This is an issue with the documentation.
According to the gnumeric manual section 10.3.15, the Surface type of plot has two subtypes, namely plotSurface and plotXYZsurface.
The code, however, implements a third subtype, namely plotXYsurface, which is completely undocumented AFAICT. It has been this way for years. If this is an actual feature that actual users are expected to use, it would be nice to see a few words about what its inputs and outputs should be.