Bernard - Omniscope does support evaluated, parametric or variable-driven field names in formulae using the REFVAL function.
That said, however, if you change the data orientation using a DePivot operation, the horizontal fields X1, X2 etc will become vertical Pivot Series and their Pivot values, including the other Y value should be included in the vertical Pivot Values field. Changing everything to row values would givve you even more options and make the problem more tractable...