Split out the looping over frequency and offset into its own function, in function of specific_analysis.relax_disp.data.plot_disp_curves().
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].
