surface

The plot style with surface has two uses.

In 3D plots, with surface always produces a surface. If a 3D data set is recognizable as a mesh (grid) then by default the program implicitly treats the plot style with lines as requesting a gridded surface, making with lines a synonym for with surface. However the command set surface explicit suppresses this treatment, in which case with surface and with lines become distinct styles that may be used in the same plot.

If you have points in 3D that are not recognized as a grid, you may be able to fit a suitable grid first. See set dgrid3d.

In 2D polar mode plots, with surface is used to produce a solid fill gridded represention of the data. Generation of the surface is controlled using the command set polar grid.