Question: Visualization of Function with Domain Dimension > 2

Hy !

I have following question;

I have some data values coming from a function of type (x,y,z) -> w.
So, the domain values (R^3) are stored in an array of the form
double[3][MAX] where MAX is an Integer. The range values are stored in an
array double[1][MAX].

The visad.FunctionType is construced with following code:
 
RealType x = new RealType("gridx", null, null);
RealType y = new RealType("gridy", null,null);
RealType z = new RealType("gridz", null, null);
RealType w = new RealType("gridw", null, null);
RealType[] xyz = {x,y,z};
RealTupleType domain = new RealTupleType(xyz);
FuntionType grid_type = new FunctionType(domain,w);

Now I construct a Irregular3DSet and a FlatField:
Irregular3DSet ds = new Irregular3DSet(grid_type.getDomain(),threeDsamples);
FlatField f = new FlatField( grid_type , ds );   
f.setSamples(rangeSamples);

Over a reference Object (DataReferenceImpl x.setData(f)) i add a Reference
to a DisplayImplJ3D.
The Scalarmaps are added to this display:

display.addMap(new ScalarMap(x, Display.XAxis));
display.addMap(new ScalarMap(y, Display.YAxis));
display.addMap(new ScalarMap(w, Display.ZAxis));

Now, my question:
How can i set the variable z to some fixed value and draw
the surface (x,y)->w ? (Note that i have to interpolate my given
datapoints.)

Thanks for every advice.