Fix faces for unstructured grid

I’ve created VTK dataset but only grid edges are visible:
plot.vtk (21.9 KB)

With Delaunay filter it’s fixed:

As I’ve checked in ASCII PLY output the original VTK has no faces (“element face 0”) while Delaunay processed VTK has them.

How could it be fixed in the input file without Delaunay filter?

Here is the VTK generation code:

         for (_xs,_ys,_zs) in zip(xs,ys,zs):
            # need to have 2 point or more
            #if len(_xs) <= 1:
            #    continue
            for (x,y,z) in zip(_xs,_ys,_zs):
                pointId = vtk_points.InsertNextPoint(x, y, z)