Skip production of plotting r1rho r2 as function of effective field w_eff, when spin.isotope is not present.
This can happen when it is 'exp_fit' model curve fitting.
sr #3124(https://gna.org/support/?3124): Grace graphs production for R1rho analysis with R2_eff as function of Omega_eff.
sr #3138(https://gna.org/support/?3138): Interpolating theta through spin-lock offset [Omega], rather than spin-lock field strength [w1].
