To use negative values you have to use a .js i believe. Just create a function with CF.setProperties to manipulate the position of what you want.
Something like:
function setPosition(){
CF.setProperties([{join:yourJoin1,x=-50},{join:yourJoin2,x=-100}])
}
setPosition();
Call the function in the CF.userMain or inside the .js, to positioned the object when your .gui load.