c++ vtk xml writer (vtkImageData) - multiple variables in the same vti file

I am trying to write data from a uniform grid to a vtk XML file, so that I can render it using Paraview. I would like my output file to contain two variables: one 3-component vector and one scalar value at every point. I have managed to write a .vti file, containing the vectors only, like so:I am trying to write data from a uniform grid t