gnuplot> set data style lines
gnuplot> plot 'runname.fields' u 1:4
(This means to use columns 1 and 4 for x and y axes.)
column 1 is theta
column 2 is theta_0
column 3 is k_theta
column 4 is Re(Phi)
column 5 is Im(Phi)
column 6 is Re(A_par)
column 7 is Im(A_par)
column 8 is Re(delta B_par)
column 9 is Im(delta B_par)
column 10 is shifted theta (only for nonlinear runs with theta connections)
There is a blank line in the *.fields file separating different k_theta, theta_0 modes, which gnuplot interprets as starting a new curve.
You can get a normalized plot of the eigenfunctions by choosing
write_eigenfunc = .true.
in the gs2_diagnostics_knobs namelist. The output file is runname.eigenfunc. The columns are the same as in runname.fields.