Using visad logarithm

hi all:
 
  i have a display of  spikes which are all clustered to some area in the
display box and i want to use the
log scale to spread it out. so it can cover most part of the grid. now the
useage is what i might need a little help with, like this is what i 
 have: 
 
 i have a math type like: (mass,pi)->ratio 
 
     RealType mass = RealType.getRealType("mass"); 
     RealType pi = RealType.getRealType("pi"); 
     RealType ratio = RealType.getRealType("ratio"); 
     RealTupleType cartesian = new RealTupleType(mass, pi); 
 
 FunctionType Ftype = new FunctionType(cartesian, ratio); 
 
  i want my x-axis to be mass and y-axis to be pi and z-axis to be ratio. 
 i dont want to take the log on the z-axis(ratio). 
 
 i am not sure if the above function type is the same as 
 
 FunctionType Ftype = new FunctionType((mass,pi), ratio); 
 
 at what point should i take the logs on the mass and pi ? 
 and how  will it fit with the data reference and the display object.

 i will be glad if you can show me some code snippets. 
 
 thanks in advance: 
 
 Isaac